I noticed that compilation at GameStop but haven't picked it up yet because I've never been a huge Sonic fan. I've heard Sonic CD was a good title though. Exceptionally memorable aspects of the game Torgo?
The neat aspect is that while traversing through the levels you can activate these kind of sign things that will either take the level you're in back into the past or into the future. Sometimes, you need to go back and forth between them to get past obstacles in later levels. The way they change the look of each of the levels depending on if you go back or forwards in time is really cool.
It sounds kind of confusing, but it actually work really well. The only small gripe I have about the Gamecube version is that the control doesn't seem to be quite as dead on as the Sega CD version, but it's still good.
I should mention however that my favorite Sonic game to date is part 3.