The turn off for so many people is the night time levels, otherwise if it were just day time levels, Unleashed would be the best since... well... since ever.
All Sonic games have something holding them back though, like SA1 you would have to play with Big and other characters other people won't like, and then in SA2 they had to make you play Tails and Knuckles along with Sonic's levels, not to mention out of the whole dark side story, Shadows speed levels only count up to 4... Heroes... it's pretty much playing through the same level with the exception of the Chaitox (SP?) where you have to stop every corner to make sure you didn't miss things you were looking for... Don't get me wrong, they weren't unplayable like some of the more recent things for some people, but had they just focused more on Sonic, then there would be nothing wrong with them...
Ah well, like Camalange said on his review of Secret Rings, it gets really good later on when you unlock everything in the Sonic game/s and replay it, but it's really tedious when you're trying to get there.