First of all I want to congratulate you guys, your game is excellent. And thank you for making it!
The first 30 levels are free, then you can unlock the remaining 170 levels via a 99c/69p IAP. After I completed the first 30 levels I couldn't smash that button hard enough, the game is worth so much more. And that's the only IAP in the game.
The puzzles are very well designed, adding new ideas as you go along so as to keep things fresh. The UI in this game is one of the best I've seen, just very cool the way simple things like the levels, achievements and even the credits are presented. Excellent stuff. I'm playing on iPod Touch 4G and having no problems at all, thanks for that, too.
A couple of things I'd like to mention. Over 200 levels I can see the robot voice becoming annoying for some. It's fine for me, he comes out with some pretty funny stuff, but over 200 levels some might want the option to turn him off.

There were times where I wanted to zoom out so that I could see the locations of the crystals and exit, so I could better plan my route. And maybe if possible add undo button? I can see later levels being quite long, and when going for a perfect score it could be annoying having to restart because of one wrong move.
All in all, P.R.O.B.E. gets top marks from me and is quickly becoming one of my favourite puzzle games. Highly reccommended.