I had a PF9 and still have a Sub2000 (G17 magwell). Both have been excellent guns for the money...and that's exactly what Kel-Tec is for the most part...a sorta cheapie brand, that offers good value.
You can do better than most of their pistols when it comes to quality and finish. But it's tough to beat the level of performance and concealability they offer for (in most cases) about $300.
Now the S2K is a fun gun that I'm going to have for a long time. Probably would make for a versatile little SHTF survival rifle as well. But no way would I ever consider spending more than $500 on one.