An overall impressive addition to the series, but not without its flaws...
The Good:
The daytime levels capture a bit of the old and the new; they truly have an awesome feel to them as you're running at high-speed with numerous routes and secrets on each stage. Though most of the time the game is in 3D, there are small sections of 2-D gameplay in the levels that bring you back. These transitions are smooth and well-done. The boss fights have the same kind of nice, epic feel to them as well. The controls for the game are also-well done, and feel fluent and natural. The graphics are well-done, and the stages have a very pleasing look.
The Bad:
Although the night time levels are a refreshing twist and are fun at first, by the end of the game with a roughly 3:1 night to day level ratio, you'll be trying to cut through the night stages just to reach the daytime levels (unless you're weird). It's not so much that the levels aren't fun or well-designed, but there's a definite lack in variety. The platforming on these levels can also be tedious or frustrating. The dialogue and such are also written at the level of a five-year-old; and to add to your misery, you can't skip the cutscenes to get on with playing.
Overall, the game is highly enjoyable, but I still don't think that it has reached a level of awesomeness that the old ones did. If they made a new Sonic game that had all levels like the daytime ones, however...this game would pretty much be a perfect 10.