Great game - so much for working today ;)
Very rarely do I play a game as well made and addicting as this. Yes, you are right - you did a great job, and it's keeping me busy for hours.
It reminds me of an old "Sierra" game called "The Incredible Machine " (remember Sierra - they wrote interactive game series for DOS like "Kings Quest" and "Space Quest" - go ahead , Google it..). Anyway, the problem solving with the available parts is a perfect combination of logic and skill - really nice.
IMHO, if you want to really put it over the top, here's some ideas you may think of in the next release:
1 - "Red Herring" parts - not all parts supplied needed to solve the puzzle
2 - As the levels get harder, maybe change the colors and background sounds
3 - Magnetic traps / accelerators that you have to avoid or use to solve a level
4 - "Nudge" keys (like a real pinball machine) to affect the ball while in play
Again, just little things - and you may have better ideas than those :)
Oh, and maybe, if you get a chance - how about a level-builder toolbox so we can make our own levels and challenge eachother?
Keep up the great work!