Updating, since I indeed got this for Christmas and already beat it (there was a long stretch where not much happened today plus I got three other games anyway that'll occupy my time). Also it was a short game to begin with.
My initial reaction was right: It was about half good, half bad. Every single Modern Sonic level had fun moments, but plenty of frustration. Well, frustration from various factors; either too simple level design or stupidly-placed objects or cheap layouts. Most of the Classic Sonic levels (
most) are okay, but the feeling of Classic Sonic evaporated immediately upon him learning homing attack after the first three stages and bosses. It... was a stupid choice, but then again, it's Dimps. It would be fine but the level design even in the Classic Sonic levels later on forces you to use the homing attack to get through. So it sucks even more; what was the point of being Classic Sonic if he has moves exclusive to Modern Sonic?
Regardless, the boss fights for the most part were pretty fun, but there's only four; the rival races are incredibly stupid and feel like a waste of time, especially since they're required to beat before you can fight a real boss. The story is even lamer than the console, and the cutscenes are terrible, considering what the 3DS can do. I admit some parts (the story) felt rushed about the console version, but this title felt even more rushed. And no, those weren't Rush puns, but speaking of, at least this game's better than Rush. Advance is a better title though, I feel. So yeah, I wouldn't buy this game until it dropped in price. But at least the final boss was way more interesting than in the console.
Haven't tried missions or online mode yet, though. I will at some point. Probably soon, but I got other and better games to play as well.