The first six levels were great, very creative premise, and it does some pretty cool stuff with it. I really liked the level where it's the same as the previous one, but it forces you to realize you can use negative numbers instead.
After that it kind of loses steam for me. The platformer level was just annoying, I had to use tabbing and the enter key to generate the blocks on screen because clicking on them wouldn't work. The memory level isn't really a puzzle at all, it just made me open the game again in a new tab. And while the music in the darkness level is pretty cool, having to solve an easy puzzle in low visibility isn't really all that fun.
The core mechanics of the game are good enough that you don't really need to spice it up any. If those last few levels just built more onto the things from the first bit, this could've easily been a 5 in enjoyment for me.