(I've had it for quite a long time, but I still go back to it now and again)
Although it's nowhere nearly as good as the 2D games or the two Adventure games on DC and GCN, Secret Rings is the best 3D Sonic game of recent years (although Unleashed may change that), having only minor issues with it.
Firstly, it's flaws: The only real issues I've ever had with it is the backtracking (camera and control), although I've grown used to it. A few abilities you can give Sonic can make him very hard to control and some of the missions can be very unforgiving if you don't have the right abilities equipped, but these are all fairly minor. Multiplayer has its highs and lows, but considering that this isn't a party game, you can't complain.
The story, style, unlockables, extras and 'feel' of the game are all handled well, as is (for a change) the voice acting. We may not have any of the original voice actors anymore, but the new ones are improving...
I overall rate Secret Rings rather highly...even if I'd rather pop a demo of Unleashed or SA2B into my Wii sometimes...