I got the game just recently, I think it's awesome, and I play it frequently. However, I don't disagree with Yahtzee or anyone else on the forums. The adventure mode is way too damn long and repetitive, half of the game needs to be unlocked, and some characters can easily win a game by mashing random buttons.
I think it's perfectly OK to use characters like Snake and Sonic in Nintendo games. However, I do see two big problems with this.
One, as a big fan of Nintendo, I really hoped that the first big game with Sonic and Mario together would've been a platformer that would be the best in the existence of the world. But no, the only games so far with the two of them together are Mario + Sonic at the Olympic Games, which more or less was a crappy Mario Party ripoff, and Brawl, which, instead of them working together in a dream team for the ages, has Mario and Sonic beating eachother's skull in with Beam Swords. COME ON NINTENDO AND SEGA, WORK WITH US!
Two, I'm deeply afraid that because two non-Nintendo characters got into a Nintendo game, the floodgates have been opened wide. Who's next to join Nintendo? Mortal Kombat? Final Fantasy? Batman?