I've just played this, and it's pretty good (mostly in terms of level design). But there's still problems regardless. This is the review of the boss acts. The other acts will come later:
Your bosses' gimmicks were creative in concept, but most of them made the fight either near-impossible to play, or just they didn't worked at all.
-Sunshine Atoll Zone 3 was pretty nice playing. The only problem I have though is that Eggman can shoot you while you're below the leaves, and that can be cheap since you don't see him shooting the laser.
-Eruption Conduit Zone 3 had it's gimmick really minor: You just could stand on a corner and hit Eggman while clearly forgetting about the lava falls. Though if you were fighting an Egg Slimer and if there were more lava falls, then maybe the gimmick would've been less forgetful.
-Drowning Downtown Zone 3 is probably the only Boss act with nothing really bad about the concept or the level design. The problem it has is it's lack of rings, but that's just it.
-Snowcap Nimbus Zone 3 is just annoying: You fight the Egg Slimer in an icy floor, with very few rings to help you fight against Eggman, ice AND slime. And the bouncing walls just don't help either. Add dry land where the player can concentrate on it's next move and add MORE rings.
-Gritty Columns Zone 3 had it's camera broken; like, if you stepped a bit in the main platform, the camera goes above you for no reason. And the quicksand is more of a pain than what it should be thanks to that camera. There's also a lack of rings (AGAIN). Fix the camera at all costs, if possible, and add more rings.
-Fume Shaft Zone 3, to my surprise, has more things than meets the eyes, at least when you see the screenshot of it. There are only 6 rings, which doesn't really help with a spiky Egg Mobile in a Techno Hill Zone 3 level design rehash, which has it's platform smaller for no reason, yet again. And a false jump or getting hit can easily knock you out of the shaft (if that's what that platform is called) if you're not careful, and you have few to no possible chances to get back (you surprisingly don't die if you hit the rails, but YOU DIE if the invisible wall gets you). Now really, I don't mind if you rehashed the level design of THZ3, but what I do mind is the size of that shaft. Please make it bigger and put more rings.
-Liftoff Gantry Zone 3 suffers from another lack of rings and lack of space to run. Please make the elevator larger and have more rings.
-Spacewalk Zone 3 is one of these Boss Acts where the concept was nice and original, but failed miserably at it. You're fighting Eggman upside down in a platform where you can barely see where you're walking, having then more chances to fall off and, yet again, with very few rings. This here can't really be helped, as far as I'm concerned with the engine, but IMO, I think it would be better if you alternate gravity. And since there's already a platform "below" the one you are in it, you just need to do something where, after some time, you change gravity. And that could probably make this fight more better.
-Alien Armageddon Zone 3 could be better if you just could take off these pillars and make the platforms more bigger. These are the only problems I have for this act. The concept here works, there's nothing really bad about it, then.
-Star Showdown Zone has the same problems as GCZ3, but just much more worse. So worse that, in fact, you can barely fight this Brak Eggman. Again, you must fix this camera problem. And if you can't, then you might as well scrap it (on the contrary to GCZ3, where it's still playable to a certain extent).
And here's the note I gave to these acts.
SAZ3: 7/10
ECZ3: 5/10
DDZ3: 8/10
SNZ3: 4/10
GCZ3: 7/10
FSZ3: 6/10
LGZ3: 7/10
SZ3: 3/10
AAZ3: 6/10
SSZ3: 4/10