OK, after I downloaded it. I've been playing for the past two hours.
Let me be clear... If you are going into this game thinking it's going to be a cakewalk like Episode I was, don't. The game is deceptive, it will not hold your hand. Some situations require minimal margin of error, not as cheap as the token massive bottomless pit in some S4:EP1 stages, though. Same goes with Sonic 4:Episode II's bonus stages, the first ones seem deceptively easy... like Sonic 3D: Blast's special bonus stages.
Oil Desert Zone which I haven't finished yet (haven't beaten the boss yet, not because I can't, I just don't know how to fight his second state), is already proving to be a very challenging level.
The bosses, the few I've faced thus far... Man, it feels like I'm fighting the final boss from Sonic & Knuckles every single time. They are being really creative with some of these bouts.
Now the visuals, Sonic 4:Episode I has some visually stimulating stuff, but this one... It's mesmerizing. The Christmas themes in White Park, the glimpses of sunlight and the waterfalls in Sylvania. Even Oil Desert has it's charm.