I'm so glad I'm not the only one stuck on that level! I've been playing both Medium 2 and 3 for weeks, and not getting past them. I could go on to level 4, but I don't want to leave too many unsolved before going on -- if not just for my pride.
In terms of stability, so far it's been pretty good, but there are times when a) so much is going on that the framerate drops, and the game becomes sluggish (which makes it a lot harder to place new towers in the middle of a complex wave), or b) when the bouncy board distortion explosion effect messes up the game, and nothing responds to your touch for a few seconds. Has anyone else had this problem?
But yes, overall, a great game both mentally and aesthestically. I'm looking forward to an update to get rid of some of the bugs, and hopefully add support for saving a game when you get a phone call/quit to do something else. Fieldrunners has such a great state saving mechanism (even if it crashes, it resumes exactly where it left off); it would be nice to see something similar implemented in geoDefense.
Take care,
Sojourn
P.S. With regards to price, I got this game when it was on sale for 99 cents, but it's definitely worth buying.