If you loved the first How to Train Your Dragon, you'll absolutely love this movie. If you are a frequent user of Tumblr, you'll want to capture SO many moments in .gif format. If you've never seen the first movie, you'll still love this movie, or at least, like it.
Essentially, this movie is great. It has all that a child could desire in a film (trust me, the kids in the theatre LOVED it - it was super cute) and it also had some absolutely hilarious moments for the adults. The plot is pretty good; even if it is predictable, it's predictable in a way that works. The animation is gorgeous. The voicework is awesome. The way all of the characters have grown over the 5 year gap between the two movies in the series was really well done and evident, not just in appearance, but also in their personalities and interactions with each other.
In any case it was a lot of fun! Go watch it, even if you don't have kids.