Absolutely a blast to watch. If you liked the first movie, you'll LOVE this one. If you thought the first was mediocre, you might still like this one.
Biggest difference between the two is the tone. The first movie was essentially a road trip movie that just happened to star Sonic. Whereas, this movie actually leveraged its source, and felt like the game. It featured locations and mechanics that calls back to the game, which will delight longtime fans. But it doesn't alienate new viewers, as everything is within the context of the universe it created.
The new characters are also a blast. Tails is the cutest thing ever, and Knuckles is a menacing yet sympathetic antagonist. Carrey is even better as Eggman this time, as he takes the physical transformation into madness.
There are absolutely flaws in this movie, and it isn't high cinema. However, the movie will delight everyone, and is surprisingly touching. Highly recommend.