Thanks for all the suggestions and comments.
Inspired by Cedrics comment, I've modified the game (just uploaded it) so that it can be played by up to 4 players simultaneously.
It now also gives you a score based on how quickly the level is solved.
I've also added a 7th (even more demanding) level and some startup-interface using the G4P library.
As for the general difficulty of the game: It becomes easier over time
I have to know, bet who played the levels XXX times to make sure they work (and to nudge things around)
But if people would like to help me designing new levels, just drop me a line (or PM)...
@pwn monkey: I've never heard of a game called "gravity hook", could you point me to the proper place? I don't claim to have "invented" this game though, I think the ideas are pretty basic and might have been used in many other, older games (?).
@yelele_eu: Thanks for the link to "string theory". Haven't heard of that one before either. I like it's simplicity. Very inspiring.