That's not to say it's not worth playing - it's a fine addition to the Sonic library, but I'd say it's not as good as Sonic Rush or Sonic Rush Adventure.
Being episodic, the game has only 4 zones, but each has 3 acts and a separate boss battle. The zones are basically Green Hill, Casino Night, Labyrinth, and Metropolis, but with new names. There are no new enemies - just ones from the first two games. The stages play almost exactly like the old ones, with the exception of a fun card-based stage in the casino zone, where you run through, on, and over cards which reward you richly with coins and extra lives. You have stage select access after you beat the first act of the first zone, which makes the game somewhat non-linear - but you'll probably end up playing it from start to finish anyway.
The special stage is from Sonic 1, only you rotate the stage itself instead of moving Sonic around. I preferred it the old way. The final boss (spoilers I guess) is Mecha Eggman from Sonic 2, only it drones on for about 4 times as long and throws some very unfair punches your way.
As for the misunderstandings: it seems like the entire team that put this game together forgot that the original Sonic games were all about momentum - when Sonic starts rolling or flying through the air, you don't need to control him to keep moving him forward. Instead, this game has you basically holding down the D-pad at all times in order to get Sonic to move. Otherwise, Sonic rolls to a stop extremely quickly. That's not how the game is supposed to play. Not even the Sonic Advance or Rush series made this mistake.
Sega might have changed the physics around because of the addition of the homing double jump, which is really a game-changer, I guess you could say. You use it all the time. You have to. If you don't, you're in danger of falling down a pit and dying - and there are a lot of unexpected pits in this game, echoing the poor level design rampant in Sonic Advance. There are some truly frustrating parts that took me 5 lives to get through, when they really shouldn't have.
There's no voice acting in the game, which is intentional - it's supposed to appease old-time fans (duh, the entire game is nothing but a giant appeasement), but there are times at which it almost seems as if Eggman is having a dialog with Sonic, where he flails around for about 10 seconds and you have to wait for him to finish not saying whatever he's saying. It's a little awkward. The music is also kind of awkward - you're supposed to be reminded of the synthy stuff from the Genesis titles, but I'm just not feeling it. It could have done better with a recorded soundtrack, or more realistic instruments. I guess I should just be happy that it's not another rap-rock debacle.
If you're a long-time Sonic fan, I'd say get the game. $15 (Wii) is a little steep for a downloadable title, but it's a nice thing to have in your collection. I think I'll get episode 2, but I truly hope they'll work out the physics and level design problems - not really holding my breath, though.