I missed the first How to Train Your Dragon at the cinema. Dreamworks' so-so record on films meant I missed it and waited for DVD. I kicked myself later when I saw it as it's one of the best animated movies of the past 5 years, and better than the last few Pixar movies.
How to Train Your Dragon 2 is a really good sequel to the first film. I don't think it's as good as the first one. It's visually more impressive with even more dragons, even more detail, but I think the story isn't quite as strong. And novelty always counts for something. But if you want a good film to keep the kids happy, or just like that sort of films, I thoroughly recommend it.
(note: we saw it in 2D, but I think it might be worth the extra money for 3D for all the flying bits).