I don't own one but have shot a few of some friends who have one. It worked well under normal firing conditions, relaxed shooting in other words. I know they prefer brass cases over steel, my father has one and after some break-in he said it fired steel fine... more or less. It was accurate enough out to 75-100yrds, about the same as my AKs actually. It would make a neat BoB or vehicle gun, it's got kuddos for accepting Glock mags... if you have a Glock that is. Like said, good luck finding one now though! I'm sure they're gone! Kel-Tec had trouble keeping them on the shelves before the buying craze...
An aside, have you considered the SU-16...? I have one and love it, it serves as my BoB rifle... well, secondary BoB rifle. It shoots all the ammo I can give it, though it doesn't like you holding or resting the mag on anything, it'll jam. Other than that, it's a nice niche weapon.