Not as bad (or good) as some people say, actually a big step in the right direction for the Sonic franchise after '06!
Finally, Sega made a 3D Sonic game with just Sonic and none of his awful friends playable. In fact, only 2 other friend characters appear, Amy and Tails. You only play as Sonic, and his stages are amazingly fun The graphics are impressive (more cartoonish but better than the poor "realistic" edge that was attempted with more recent titles) and Sonic handles well. He's fast, and some of the new moves aren't awkward. There are still super speed-esque sections from the last game, but they are much more fun. Rather then dart forward, you have more control over Sonic's movements and speed, preventing the absurd difficulties of '06. There are also points where the 3D view switches to 2D, a very welcome throwback. Whether its speeding through stages or beating bosses, you really get the sense that Sega has started to perfect what a Sonic game should be.
Note that I said "started". There are some 10s and 9s from reviewers on this site, but the game isn't THAT good. You may have heard of the werehog, an awkward take on Sonic, because as much as people wanted a game with just Sonic, you know Sega couldn't just have the game be about speed. The werehog stages are actually fun at times, but they usually become very tedious after a couple of minutes. Its pretty much just a button masher when you're playing as the werehog, which isn't what Sonic is supposed to be about. When climbing poles, Sonic will often get stuck and you may have trouble detaching. The autolock will occasionally not detect poles or objects your supposed to swing with over bottomless pits, which will result in your death. You are also painfully slow as Sonic the Werehog, making what are actually short levels seem very long. Still it can be fun grabbing and beating on enemies, especially as you level up and they are helpless against you. For the most part though, the Werehog is a disappointment.
You switch b/t the Werehog and normal Sonic depending on the time of day. The game will give you the option to switch, but unfortunately, which form you have to take to progress is locked for you. For example, you may switch to daytime but when you reach the area where you have to start, you'll find that the next mission has to be completed as the werehog. You also have the ability to level up based on exp gained. Experience is gained from beating enemies and eating food that you buy (from stores, another neat feature). This allows to build your character up, making the game more fun as you progress. Town stages also return, but they're better than in '06. They're small, you don't have to really talk to many people, and there are optional side quests. At night, you can excorcise demons from people (part of the plot), though this leads to a short Werehog mission, which is somewhat boring. The story is good, though somewhat similar to previous games (Robotnik trying to release some ancient demon again only for it to backfire eventually), but Sonic's quest is pretty original.
Overall, it's a good game, but not the full revival of the series that Sonic needs. However, it is a step in the right direction and gives me optimism that Sega knows they need to start fixing things to bring Sonic back to the list of great games. If your a Sonic fan, its a must-buy, if not rent it first and decide for yourself.