It's not your usual Sonic Game, but that isn't neccessarily a good thing
The gameplay in this game is an altered version of the usual formula used by Sonic Adventure and Sonic Adventure 2. The handling is very loose, even more than Sonic Heroes, meaning it's very easy to lose control no matter how fast or slow you're moving. The gun-implementation was only half-successful. While it does offer rapid attacking, picking up/swapping guns is extremely difficult to do, especially when you need to hurry, not to mention the fact you tend to run out of ammunition at the most inconvenient times. If you are adept to Sonic Adventure 2 however, you can get relatively far by using the normal Sonic-gameplay, but you will have to use guns every now and then.
Control is a mixed issue. While it is half the same as Sonic Adventure 2, as previously mentioned, it's very loose. Holding a gun will render you unable to attack normally, thereby wasting ammunition or risking dropping the gun. Had this game been made without the possibility of falling off the edge, or the "snap-decision" speed, the controls would be much more effective. The vehicles are a waste of time, and therefore should be ignored when the opportunity to use them arises.
Sound, this is the one thing they managed to pin correctly. The music is great, fits the game very well with a dark, metal-appeal. Crush40 returns with its usual amazing lyrical-tunes to further add appeal to the game. Unfortunately, the absence of Ryan Drummond and the rest of the cast from the previous games is an immense loss, replaced with the often eye-rolling cast of the 4kids-dubbed "Sonic X". You tend to get tired of Shadow narrating his every action/plan rather quickly.
Graphics, it seems as if it's been downgraded a bit from Sonic Adventure 2: Battle. The movements aren't as fluid as they previously were, although that only applies to gameplay-graphics. The intro-movie is amazing, meshing perfectly with Crush40's "I am". The gameplay rarely lags, although the constant motion and explosions will make it very hard to keep a clear view of what's going on.
In the end, if you're a true Sonic fan with alot of free-time on your hands and SA/SA2/SH weren't a challenge, then this is the game for you. Otherwise, it is an experiment that could have gone better on Sega's part.