Graphics, Music, Chao Garden were amazing.
Multiplayer was pretty awesome.
Had fucking SECRETS, including hidden paths/flutes - an amazing feature given the trend at the time of its release.
Sonic/Shadow levels were easily among the most fun I've had in a Sonic game - the only 'superior' levels being some select stages from the Sonic 1,2,3,S&K era.
The game is unfortunately tainted by the hideous 'Sonic friends' stages. They are not broken - and on some level, can be mildly enjoyable... but there is a such a massive discrepancy between the quality, design, replayability/exploration and music of the Sonic/Shadow stages and the 'Friends' stages, that it is impossible to forgive. The Tails/Eggman stages were slow and tedious, but ultimately short enough and simple enough to satisfy as 'variety' in the game. The Knuckles/Rouge stages were nearly all awful however. Confusing level design, clunky movement, terrible camera, awful music and an overall concept that didn't really fit the Sonic design guidelines at all.
For anyone on the fence, I'd recommend watching some Youtube videos of the Sonic/Shadow stages and see if it looks at all entertaining to you. I replayed some those stages to death, Radical Highway, Skyrail etc. were all amazing. The 'Friends' stages are a mild annoyance if you enjoy what the game has to offer at its core. If the Sonic/Shadow stages don't interest you, avoid at all costs. The 'Friends' stages will drag the game out for you until you hate it.
As for my view/judgement of the game. I fucking loved it when it came out. I played the living shit out of it, and once I had beaten the game never looked back at the shitty friend stages. I could have listened to the soundtrack all day long, and the multiplayer was surprisingly enjoyable. I tried it again recently and still thoroughly enjoyed the game until I reached the Tails/Knuckles stages. I ultimately tortured myself through those stages so I could reach the later stages/ending. I'm not sure it was worth it, as those stages truly are awful, but there is some payoff with some of the later Sonic/Shadow awesome-ness.
I'll buy it again just to show Sega that I would still like a 3D Sonic that isn't the awful shit I've had to witness over the past 6+ years.