Sorry to hear the crankset isn't working out for you. I have just over 400 hard miles on my 2010 k-force crankset, and no problems at all. The pre-tension nut on the non-drive side did need to be tightened once, but that's the LBS' fault, not a product issue.
The chainrings don't seem flexy to me - I'm not sure if it's because my other cranksets have potentially been flexy so I'm used to it, or if they changed the rings since 2007.
I was hesitant to put carbon cranks on a trail bike, but after talking with several people, my concerns vanished. I believe I found the information in threads here regarding older models having issues with the pedal inserts. If my set develops any issues, I'll be sure to post about it here.
For anyone who's wondering, the crank arms handle the inevitable rock strikes just fine (I live in New England where it's very rocky/rooty). The ends of the arms are solid carbon, not holllow like the rest of the arm (this may be a change that's eliminated the loose insert issue - not sure if the older models had solid carbon around the inserts).