I have the iPhone version so far (for free so I could judge if I'd want to waste $15 on it for the Xbox 360 or not), and it's only really the PartnerNet build from earlier.
Still very very fun for what it is (knowing it's had a good bit of improvements since the delay). The game's physics aren't as bad as I heard people say they were (even though they could still use some fixing), and it doesn't feature all the polish of what the console version supposedly has. Minecart level isn't as bad as I thought, but the score attack level is pretty obnoxious. I'm bothered that I can't progress directly through the stages in order like we could on the console edition, but I'll live until I get that one on Xbox Live in the next two days. Also, homing attacks and dash panels are alright with me. They don't seem to invade the gameplay too much which is much more awesome than I thought.
Also, relating to the physics issues after playing... running and spinning physics seem reversed.
In the classics, running speed wasn't as highly affected by slopes and was somewhat capped, while rolling was heavily affected by slope steepness and has no cap. This is awkward in Sonic 4 because of how for some reason it's reversed where running has no speedcap and is affected by slope steepness, and rolling isn't either.
Also, Special Stages = HARD.