I nearly beat level 5, but when I got the jewel on the far left and couldn't get back to the chest I had to quit. This game is just too damn hard.
The mechanics feel a lot like a vertical Meat Boy. Jumping and stomping are extremely fast, as opposed to MB's super fast horizontal movement. This gets really interesting with the relatively slow falling speed, as a LOT can happen while you're airborne and this makes sticking landings especially difficult. I sometimes noticed I landed on people but still hit the spikes anyway. Very annoying.
I would highly recommend putting in some moderately difficult levels between 2 and 3, even on difficult games you never see difficulty ramp up this quickly.
Some other suggestions: Don't restart the music when the player dies. It happens too much and players will never hear more than the first 10 seconds of any given song. Give the spikes a bit more glow/contrast with the rest of the level. You can play these levels by instinct but everyone else has to use peripheral vision, so do what you can to make that part easier for us. I also think jumps where you have to wait for certain obstacles to go in and out of phase should be changed so that the window of opportunity happens with each cycle of animation. Waiting for two obstacles to go into phase and missing the jump anyway is probably one of the most frustrating things you can experience in this game.
The graphics are interesting (especially the psychedelic effects) but ultimately pretty basic. Though for something this challenging where you stare at the same things for a long time, it might be nice to have something even simpler, or at least just more high contrast (getting back into what I said about the spikes.)
All in all the game is fun and brutally difficult, but eels more difficult than rewarding, especially as there's no real downtime where you can take some time to breathe between the hardest parts.
EDIT: So I decided to give this another go and got to level VI. I've collected as many gems as I could see (one on far left, two in the small jumps on the right, two in the big jumps on the right) and it's still saying I need one more gem. I restarted the game and my save file was deleted so I got sent back to level 1. Some kind of bug?