Initially fun, but gets boring fast
This game's biggest draw back was the fact that the gameplay was fairly repetitive and lacked any real replay value -- as the player you don't really need to master any new skills or strategies as you advance through the levels. Until the last two or three levels I barely had to change layers at all; on the few occasions when an ash formation got in my way and would've forced me to switch, I usually had a nice arsenal of the special weapons built up so that I could just destroy the clouds instead (which is really the better choice anyway, since you get huge amounts of money for destroying the clouds).
The controls were decent and fairly responsive. However, as has already been pointed out, there was extremely noticeable lag when switching between altitudes. This became particularly problematic after grabbing a few of the speed-increasing pickups, since they made it even harder to react effectively to the slow-changing layers of ash clouds.
The Shop could also use some descriptions of what the various purchasable upgrades actually do -- after having completed the entire game, I'm still not sure what spending all that money actually did for me.