I've had my RFB for about a year now. I got one of two that have come in to H&H Firearms in Fort Wayne. I heard Kel-Tec has ramped up production on these now, and hopefully availability will get better.
This is the finest piece of engineering and machining I have seen from Kel-Tec. The weapon fires and functions flawlessly and is just tons of fun to shoot. The .308 RFB is actually shorter than my FN FS200 and MSAR (both of those are .223). The RFB uses FAL metric mags which are easy (and cheap) to obtain. I got 4 Israeli mags produced in 1976, in the wrappers and coated with cosmoline for $80.
I've shot 150 gr, 155 gr and 168 gr ammo (factory and reloads) without any adjustments to the gas system and haven't had any fees or functions issues.
If you can find one, grab it up and have a blast.
Now that's what I like to hear have you shot any surplus or steel case ammo through it? I'd also be interested in how the trigger pull is, heavy or long? Can fast followup shots be taken compared to the 5.56 bullpups?