I have recent experience with it. I started with the newer series, the one that has run for 2 seasons and for which they plan one more. I liked it quite a bit. Both leads are solid. Season 2 was more interesting for me than season 1.
Then I moved back to the original Discovery Channel series. It has higher highs and lower lows. They take more creative risks, but it can also be cheesy. Most fans will tell you that the show drops off after Ragnar Lothbrok is no longer on the show. For me, it dropped off heavily while he was still on the show (season 3) and then rebounded well for his final season. I didn't make it very far into season 5, the one after he's gone.
Overall, I think it's mostly entertaining. As long as you don't go in with expectations of it being a high tier drama, you should have a good time.