All of the 3D Sonic games have been underrated by nearly every reviewer, but none so viciously as this one. I won't be a fanboy and say that this game is faultless, and I won't slam it just because suddenly it's become "cool" to bash Sonic, but I will give an entirely honest opinion of this game.
Number one: Camera issues. This is one of the main things that people bring up when discussing this incarnation of Sonic, but in response I say "What camera issues?" The right stick controls the camera and sure, it's not exactly lightning fast, but the camera always looks where you tell it to look and doesn't ever flip out and do something weird which causes you to fall off - something that nearly every other reviewer says.
Number two: Loading times. I have never experienced such nasty loading times as in this game. In the main levels it's fine, no problem, but in the side missions... it's just painful. You get the mission, then loading screen. Then two lines of dialogue (which is WRITTEN, not even spoken), then another loading screen. Whenever I play a mission I play my best - because I know that if I don't, I have to sit through FOUR loading screens (one to load the "mission failed" type dialogue, one to load the city, and then back in) to start again. This is just plain wrong. In the middle of a "proper" level, if you die, you INSTANTLY go back to your last checkpoint with no problems. How come the side missions can't do that? The loading times are the only major problem with this game. Seriously. But you have to just lump it if, like me, you think the rest is great.
Number three: Voices. As you all probably know, the voices since the Adventure games have changed. For the worse. Tails is painful. Knuckles sounds like he's desperately trying to be how he used to be, but failing - which makes it worse. Shadow isn't bad, but he's still not like he used to be. Guh. Bad, bad voices. Apart from Eggman's - that's actually an improvement.
Number four: Character speed. Sonic's fine, Silver's fine - Silver's supposed to be slow. But Shadow, I think, is slower than he should be. That's just me. And I miss the yellow glow that his shoes had in Sonic Adventure 2. I've never played Shadow the Hedgehog so I don't know if they glow in that, but I think they should. The other characters for the most part are an interesting change, but Tails can quickly get frustrating.
Number five: Storyline. This is the best storyline Sonic's ever had. Bar none. Although the whole retconning the entire story at the end is a bit "and they woke up and it was all a dream." I can't decide if I really like that bit, or if it just annoys me.
The gameplay in this game is just like the other 3D Sonics, but better. Silver's especially fun. Shadow's vehicles are pointless, granted, but you don't HAVE to use them apart from on about three occasions. Shadow feels powerful on his own, which is nice, because he's supposed to be. The oft-slated-on-Amazon mach-speed stages for Sonic are just like the Wii game - which gets rave reviews. So I don't know what people are talking about when they badmouth them. They're damn hard, but apart from that...
If you like platformers, and you can bear loading times, buy this game. I love it. And the music is awesome too. Especially Crisis City and the final boss - which is quite typical Sonic fare. I think, without the loading times, this could have been the best Sonic game ever, bar none. Sadly, with the loading times then Sonic 3 & Knuckles retains that title, with Heroes coming second (despite Heroes' abysmal storyline) and Adventure 2 third (the duller stages let it down).