I'm writing very detailed impressions right now. I will simply link my blog, because I spoil the entire plot up until where I stopped playing (after Crisis City), but a very simplified version is: Yes. It's shit.
It's perhaps one of the most shit games I've ever had the misfortune of playing. Sub-missions in Soleanna are horrible, the controls fight you every step of the way. The game very clearly decieves you; you may choose to play as Sonic, but mid-way through some stages, it'll switch to playing as Tails or Knuckles, and their gameplay and controls are ten times worse than Sonic's. It's a big, buggy, horrifically unpolished mess.
Mach speed stages are fucking awful, too. Hardest part of the game because you're moving SO FAST and the controls get really slippery. It becomes a matter of "If you get hit, try and find some more rings as quickly as possible before you get hit again" - and you WILL get hit again. It's almost impossible to AVOID any of the obstacles in the mach speed sections.
There are still moments when the gameplay clicks together and everything works as it should, but those moments are fewer and farther between than ever before.
I will say, the plot and the production values given to cutscenes are higher than ever, and I would enjoy this game a lot more if I didn't actually have to play it.
I've heard playing as Silver is a lot more coherent than playing as Sonic, though, but I haven't touched Silver or Shadow's stories yet.
Edit:
And here's the blog. I wrote
a lot.
Edit 2:
Crappy webcam footage ahoy!
http://blazemp3.sepwich.com/sonic2k6/so ... mecore.avi - Plot spoilers mainly, as I let the cutscene run for a little bit before skipping. (Digicam only records up to about 1:30-2:00 of video).
http://blazemp3.sepwich.com/sonic2k6/so ... ecore2.avi - Audio goes wildly out of sync on this for some unknown reason. No plot spoilers, really, just gameplay. Showcases some really stupid glitches that weren't in anyway my fault, the game just decided that instead of sending me on it's scripted pathway it'd fling me into a pool of lava.