I recently acquired a PF-9, and after solving a few minor issues, I'm very pleased and impressed.
That said, the PF-9 had some problems in early production runs, but Kel-Tec seems to have worked out all the bugs. If you're buying a used gun, that might be an issue, but Kel-Tec does stand behind their equipment, and will make it right if you end up with one of those.
After shooting a number of times, the ejector spring screw loosened. IMHO, it appears they put something like Loctite on the very end of the screw.... and it needs to be close to the head of the screw. Otherwise it will back out slightly and cause fail-to-eject problems, and if loose enough, it will allow the firing pin to fall out. A touch of BLUE Loctite will solve the problem. Again... this is a MINOR issue, and you can check the screw for tightness with the supplied Allen wrench.
If you are recoil-sensitive, this is NOT the gun to have. A friend and I acquired ours at the same time, and while I enjoy recoil, he doesn't.... and he shoots a few rounds and puts it down for the day.
Someone in a previous post had a Kel-Tec P11 and said the trigger was terrible. From those I've handled, I would agree. The trigger of the PF-9 is totally different, and while long, is NOT at all like the P11 trigger.
While yet unfired, a friend just got a Ruger 709 Slim. By weight, the 709 is about 50% heavier than the PF-9 and just slightly larger in more than one dimension. I'm guessing the perceived recoil for the 709 is going to be much lighter.
BTW, while factory loads will result in some people selling their PF-9s after a few shots, using my old Lyman reloading manual as a guide, I created a minimal load with cast wheelweights for 120 grain truncated cone bullets, and even my wife will shoot this thing. Heavier powder loads have all resulted in keyholing in both of my 9mm guns with those bullets.