Grass/Forest Dimension: It was relatively easy for a first section, but it took me a while to find the red spring and I lost several lives while doing so. Please don't hide it among the bushes, SRB2 is usually not a treasure hunt, and when it is, there's an emerald radar. The platforms that move up and down all at once took me a while to figure out, and they're rather small for their point in the level, but I appreciate the challenge. Also, some of the platforms are misshapen so the flats on their surfaces are cut off on their right side.
Desert Dimension: I'm not sure if you noticed this, but the Minuses seem to be invisible until you're about 150 fracunits away, which is kind of odd, but not really your fault. Anyway, I agree with what IE said, you should make the textures less confusing so it's easier to tell which areas are quicksand and which aren't. I don't have any real problems with this dimension other than that except that the platforms are, once again, too small. Also, in case you haven't noticed, the Starpost Activator sector in this dimension is misaligned.
Ice Dimension: First of all, NEVER put death pits where people can't see them. Those gaps between the stairway and the platforms at either end need to either be wider or nonexistent, because at their current size they're almost completely invisible to the player until they've already walked into them and lost a life. It might also be a nice gimmick to have slippery surfaces in this dimension, even though many other levels use that gimmick as well. If you do make the surfaces slippery, though, you need to make those tiny platforms bigger once again.
Lava Dimension: OMFG, Rocky. This dimension is a NIGHTMARE with Sonic, and the difficulty spike between it and the ice dimension is absurd. I lost more than two-thirds of the total lives I lost on this level in this dimension. Yes, this dimension is twice as hard as all the others combined, at least to me. Besides the incredibly annoying spikeballs on those still rediculously tiny platforms, there are also those infuriating Buzzes that always show up just when you're about to make a tricky jump and screw you up completely. If nothing else, just make those platforms bigger... D: Also, you can skip all of the spikes by walking along the edge of the platform, and the lavafall just before the spikes doesn't harm you.
Boss: A rather interesting arena, and we don't get to fight the Egg Mobile with spikeballs very often (even though a SOCed attack or two would have been nice), but I do still have two major problems with it. First of all, the arena is too large and it takes forever to lure Eggman back over to you once you hit him. Second--and this is by far the most frustrating one--there's no checkpoint right before the boss teleporter, which means that if you die on the boss, you have to do the entire already mind-blowingly cheap lava dimension over again. However, this problem is rather easy to fix. Just add a Star Post right before those springs and some rings in the boss arena, and you'll be fine.
Conclusion: In general, this was a rather entertaining level, although there are still some glaring flaws with the difficulty curve and the character balance that prevent this from being something I'm inclined to play over and over again. There were a few interesting platforming gimmicks, and you really went out of your way to hide secrets in clever ways, but this level just doesn't have enough redeeming value for me to enjoy playing it.
Rating: 3/10