I have known a lot of people who have bought karmas (my employer has a great pro-deal with Jackson Kayak and we can get them for a steal). I have paddled one a few times, but I still prefer my displacement hulls.
The feedback from all of them has been pretty much the same. Namely, that the karma paddles great, but that warranty is pretty darn useful.
Future models will probably fix some of these issues, but I have seen a lot of premature oil-canning. I also had a buddy whose outfitting came loose due to a broken plastic piece that couldn't be replaced as it was integral to the hull. Another friend of mine pitoned the hell out of his karma hitting a rock that I and many others regularly encounter without suffering any structural damage.
The karma paddles extremely well and has the right combination of features for paddling the big stuff. It boofs very easily and lands surprisingly softly for a planing jul.
However, given that this is a completely new design, Jackson seems to still be working out some kinks.
If you can buy a new one, this isn't too much of a problem as Jackson has a fantastic warranty (one friend is on his third boat in a little over a year).
I should also add that these boats probably last a lot longer if you paddle mostly I-III stuff. Most of the issues I've discussed have happened to people who regularly paddle V+ (especially the Green Narrows).