Damn hard.
I could just about finsih level one and when level two came up I was like "dude this isn't gonna happen". It's nigh-on impossible to get it accurate, I mean the controls are screwed. Also, another down-point, is the shine on the marble - it's much too harsh, that I actual didn't realise that it was a marble until a minute into it. It looks like a dummy.
Sort out the controls a little, make it less un-forgiven and Im' sure it'll be a good game. Everythings there, it just hasn't been packaged properly yet. Good luck.