Probably the best of the 3D Sonic, but I guess that isn't excavate a good sell.
It has some nice ideas, offering both 2D and 3D with each level having multiple paths to encourage replay value and side quests akin to Mario 64's star system, changing the context of levels.
The levels themselfs are decent designed providing a fair bit of challenge in later stages if you intent to go through them at some speed, which can be borderline overwhelming, but also exhilarating.
The problem, as ever, lay with controls.
Simply put, the lock-on system is terrible. The player has some control via flicking a direction, but when you want to do jumps and not actually attack an enemy, multiple times it will automatically take over your controls, forcing you on an unwanted path.
Other times it simply won't register at all. In one case, literally at the opening 10 second of a later, it refused to activate causing repeated continue screens.
At this point, Sonic Team have no excuse, they simply refuse to acknowledge the outside world instead opting to live in it's own little bubble.
Occasional camera problems rear it's head as well, where Sonic will inexplicably fall of a map thanks to an unseeable pitfall.
Speaking of which, a piece of Sonic design i've never liked is still in use, deliberately putting enemies where the player will project, with practically no way to avoid them than knowing there location in multiple play seasons.
Boss battles are, as the series generally goes, easy. Engaging enough, but not nearly as much as they could be.
Story? It has one. It's throw-away.
Sonic Generations is merely ok, by Sonic Teams standards, that's a Chaos Emerald.