Finally, Sega nailed it!
Gameplay is a blast! What made Sonic great was his speed and that's exactly what Sega has implemented not only with the classic side-scrolling style of the early days but the speedy, 3D homing-attack-style of today. Environments are gorgeous though you won't be stopping to enjoy the scenery much.
The only problems I could find would be the somewhat easy and anti-climactic boss battles. Don't get me wrong, seeing all the familiar faces from the past is great but if there's no challenge it can dampen the thrill.
There are also some points where Sonic will stop dead in his tracks by barely tapping the side of an object or wall while running. However, this can be overlooked since the majority of times you have the ability to blast by or through these nuisances at high speed anyway.
I along with many fans can't help but rejoice as to what Sega has reproduced. The nostalgia in this game is at an all time high and there's even higher hopes for the hedgehog's future. Welcome back, Sonic!