A review of Heroes of Might & Magic V: Hammers of Fate can be found at Evil Avatar.  The rating is 2/5 and the intro sets the tone:

There are two kinds of expansion packs in this world. The first adds new and interesting gameplay elements while simultaneously massaging away some of the poor design decisions in the base game. The second kind slaps on a handful of indistinct new features, most of which were probably removed from the original game in an effort to churn out a quick, cheap expansion pack. Guess which one Heroes of Might and Magic V: Hammers of Fate is?

Games Radar has a review of HoMM V: Hammers of Fate. The reviewer hands it a 6/10 and says:

Somewhere along the way it seems Russian development team Nival Interactive got its signals crossed. What we wanted after the promising mess that was Heroes of Might and Magic V were fixes, and after the fixes, an expansion with a campaign worth playing. Not just a hodgepodge of features that frankly should have been in the original or dished up alongside the patches.

IGN has posted a review of Heroes of Might & Magic V: Hammers of Fate. Sporting a "passable" 6.5/10.00 score, the reviewer said: 

Hammers of Fate offers no real surprises in the areas of graphics or sound. The visuals are still fairly sharp and the new Dwarf designs and animations fit nicely with the overall art style of the game. We particularly like the upgraded Berserker armor and the freaky flaming fists of the Warlords. The Dwarven city is similarly well executed with plenty of style and impact. Unfortunately, the game still relies on poorly staged, unremarkable cutscenes run through the engine.

Closing Comments
Serious fans of Heroes of Might & Magic aren't likely to be discouraged by the lack of new content or the thirty-dollar price tag. With only one new faction and a smattering of new gameplay options, Hammers of Fate feels more like a small-scale mission pack than a full-fledged expansion. You'll get a good fifteen to twenty hours out of the campaign but it's not any better than the user-created content that's already available for free.

Two reviews of Heroes of Might & Magic V: Hammers of Fate have turned up today. The first comes from Eurogamer, scoring the game a very low 4/10:

From all this hopefully everyone will be able to decide whether the add-on pack is what you're looking for.

Which leaves the mark, at which point the ceiling of this objective world we've created comes crashing in and we're back in the torrid pit of subjectivity again with the simple question of "Would I buy it?"

Hammers of Fate adds a lot of material for HoMM5. But to warrant a better mark, it would have to actually deal with the basic weaknesses of the game. As it is, despite the Caravan's efforts to streamline one aspect, it just doesn't.

In other words, would I buy it?

Not on your nelly.

Considering the original game only came out a couple of months back, obviously when it comes to the fundamentals, the developers haven't had an awful lot of time to make many changes. Indeed, in many ways, it feels as if you're starting from scratch with a campaign that literally sends you back to base camp. In my case, having just reached HOMMV's end-game, I'd rather have waited a bit longer and received a bit more. Led Zep this isn't, but then thankfully, it's not Marillion either.

Noting that several of the inprovements included in Hammers of Fate are available as separate downloads or patches, GameSpy's review scores Nival's HoMMV expansion at 3.5 stars.  Here's a clip:

The expansion introduces a new town type: the Fortress, which brings the grand total of towns to seven. Fortress is basically a different way of saying "the Dwarfs." You get all of the basic trappings of any town, plus a few unique abilities such as Rune Magic, which is the Dwarf way of casting spells and uses resources rather than mana points. Some of the Fortress units are pretty unique, though, and not just your basic short, bearded fellows toting axes.

Playful's has posted their review of HoMMV: Hammers of Fate. The score is 7.2/10 and here's a snip, which proves no easier to read than their last article we covered:

The reduced content of novelty dissolves quite rapidly in a mix of "beatitude" and ease in advancement. At a Normal difficulty setting, things tend to become in no time extremely easy. I'm not a player who doesn't appreciate the fair-play attitude, reserved to the first part of the script, when everyone is preoccupied with seeking natural deposits and building their own towns, but when things tend to remain fixed on the same seamless line, something whispers in my ear that it's time for a much more detailed analysis, somewhat agitated under the premise of a programming "error". Thus, the guardian-creatures and even the other heroes that call the map their home, rarely pose any danger for the player controlled hero, and even when he stumbles upon a "well-oiled" army (after several good hours spent in the game world), the A.I.'s hesitant nature offers the necessary time to regroup and strengthen the soldiers' rows. The differences between Normal and Hard become much too obvious and disturbing. In the end, Medium would've supposed an attempt to minimize certain aspects that would've pose problems to the player, but by Normal, I consider sharing the experience that the producers intended for the players to live. Very considerate of them, a little bit more tweak under the hood and maybe things would've followed a more accentuated ascendant slope. I'm not denying the fact that eventually, everything is set into motion, but it's much too late to save the appearances.

The first review for Nival's HoMMV expansion Hammers of Fate has drawn a mixed response from GameSpot and a score of 6.4/10:

Considering the rabid fan following and the long wait, it's not a surprise that Heroes of Might and Magic V was picked apart by certain members of the hardcore fan base. It was criticized in more than a few quarters for a dull campaign, some glitches with the camera system and multiplayer, and simply for being too updated a copy of the late, great Heroes III. A series of patches helped a lot, though, and the game looked on its way to being one of the best in the history of the franchise. Now, however, the series takes another step sideways with Hammers of Fate. Like the original game on launch day, this expansion pack balances good points with bad almost every step of the way, resulting in yet another turn-based fantasy adventure that isn't all that it could be.

Nival's expansion for Heroes of Might and Magic is on its way to NA retail:

HEROES OF MIGHT AND MAGIC® V: HAMMERS OF FATE SHIPS TO RETAIL

SAN FRANCISCO – November 14, 2006 – Today Ubisoft, one of the world's largest video game publishers, announces that Heroes of Might & Magic® V: Hammers of Fate, a Heroes of Might and Magic® V expansion pack, has released and will be at North American retailers this week.

Key features of the expansion pack include: • New faction: Introducing the brand-new Dwarves faction, made up of heroic warriors and rune magic adepts who stalk Ashan's deepest mountains and most hostile regions.

• A brand-new adventure: Featuring 15 new missions, new heroes, and improved cut-scenes, the new campaign will lead players even deeper into the dramatic events that began with Heroes of Might and Magic V.

• Extended game experience: Includes a wealth of new spells and abilities, 5 new single-player maps, 10 new multiplayer maps, new buildings, new artifacts, and new neutral creatures, Heroes of Might and Magic V: Hammers of Fate gives players the tools to develop new tactics and further their game experience both online and offline.

• Improved mechanics and gameplay: New features, such as Caravans and a random map generator, allow players to have unlimited replay options and an even smoother gameplay experience. In addition, the simultaneous turn feature will speed up online games.

Developed by Nival Interactive, Heroes of Might & Magic V: Hammers of Fate releases with an ESRB rating of "T” (teen) and an MSRP of $29.95.

For more information on Heroes of Might & Magic V: Hammers of Fate, please visit the official web site at www.mightandmagic.com.

A more general question now: Fans are divided in their opinions on the patches, that is to say: the game's balance etc. – but they seem to appreciate your efforts in continuing to issue updates. Do you plan to continue issuing patches/updates for the standard version also after the expansion is released?

Yes, we still haven't concluded our initial patch plan, mixing balance tweaks and new features. We've been gathering delays in this respect, especially as the addon was entering a crunch phase. In the last few weeks of patch 1.4, we have also concentrated on the desynch problems, as it seemed cases were still being encountered. Nival produced a special version of the game for our testers to track these issues, which are stemming from various and specific game situations. I hope these problems will now be behind us. After this initial plan is done, I think we will draft a new one to continue providing new content!

Mightandmagic.com announces that Hammers of Fate, the addon for Heroes of Might & Magic 5 has gone GOLD:

The Heroes V expansion: Hammers of Fate has gone gold!. A quick summary of what to look forward to is as follows:

New faction: Introducing the brand-new Dwarves faction, made up of heroic warriors and rune magic adepts who stalk Ashan’s deepest mountains and most hostile regions.

A brand-new adventure: Featuring 15 new missions, new heroes, and improved cut-scenes, the new campaign will lead players even deeper into the dramatic events that began with Heroes of Might and Magic® V.

Extended game experience: Includes a wealth of new spells and abilities, 5 new single-player maps, 10 new multiplayer maps, new buildings, new artefacts, and new neutral creatures, Heroes of Might and Magic® V: Hammers of Fate gives players the tools to develop new tactics, and further their game experience both online and offline

Improved mechanics and gameplay: Thanks to new features such as Caravans and a random map generator, players will enjoy unlimited replay ability and an even smoother gameplay experience. In addition, the simultaneous turn feature will speed up online games.

For more information check out this unofficial site which has more details on the expansion.