I've been using Metanx for nearly a year. My PN is idiopathic (at this point) but I have reason to believe glucose intolerance (not pre-diabetic, but close enough) is at least somewhat at fault.

I haven't had any pain relief from the Metanx, but it did seem to ramp up my circulation, which has always been poor to my extremities. For me the idea is that the B12 and folate at high levels will help create an environment that could be conducive to nerve regeneration. I haven't had any strange side effects so I plan to continue taking it for the foreseeable future.