Very nice
By level 15 you really start to think in a different way. You realize that the red has to go around the blue or they'd criss-cross, or that the orange has to do the snakey-thing, or that the green cannot go there because there's only room for 1, not 2 in that corridor.
I beat the game finally, got stuck on level 19 forever. But it's possible. Even level 16 ;-)
So I repeat, great game, and it is beatable.