First of all, Sonic 4 is not a bad game. It was perceived as being bad likely because it changed the formula of the classic Sonic games and people didn't like that. Had it not been titled "Sonic 4," a lot of the negative reception would be diminished considerably, since many simply didn't think it was a worthy successor to the classic games. You have to look at a game both as part of a series and individually in order to have a valid opinion of a game, and that's exactly what I plan to do in this review.
To begin with, I like the visuals and art style (although cel-shaded Sonic looks a bit odd). The music is great, despite what some have said, and the level design is definitely on par with the classic games, albeit slightly more linear. Of course, one of the major inclusions this time around is the Homing Attack, a move that many felt didn't work in a 2D Sonic game. I honestly didn't have any problems with it, I actually found it to be implemented quite well. Now onto the physics. Literally EVERY REVIEW I have seen of Sonic 4 mentions the physics being poor or "floaty." In reality, there are no discernable issues with the physics in Sonic 4, and when compared to the classic games, the physics could be considered better. I'm guessing that, once again, these people are using that same "different = bad" principle, as they have with the rest of the game.
Moving on, the bosses are generally pretty easy, although the final boss is relentlessly difficult. The Special Stages are all ♥♥♥♥ing insane, and the last one is quite possibly the cheapest, most unforgiving Special Stage I have ever had the misfortune to play in a Sonic game. I've tried again and again with no luck, so if anyone has some incredible strategy to beat it, please let me know. Anyway, the Zones themselves are actually pretty creative (especially Casino Street), but aren't really memorable, with the exception of Mad Gear Zone. That ♥♥♥♥ is epic.
Of course, as with the classic games, there are a few cheap deaths and unfair tricks in the game that can become incredibly frustrating, but nothing too big besides the aforementioned final boss and Special Stages. Overall, it's actually a decent game. Maybe it's not a real sequel to the classic Sonic games, and maybe it's flawed, but it's still enjoyable and holds up as an individual game. If you want a true Sonic 4, go buy Sonic Advance, but if this review hasn't convinced you to at least give this game the credit it deserves, then I don't know what will.