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).