As you probably know, Neurontin was originally approved by the FDA as an anti-seizure medication. Since then, it has been prescribed for other conditions, and I think that the FDA is going to (if not already done) approve it for other conditions. I was prescribed N for peripheral neuropathy. It has also helped my tinnitus, and headaches.
I too take from 600-900mg/day. At first, I kind of got a slightly goofy feeling when I took it, but now that has gone away. I have taken it for ~7 months. Some take up to 4000mg/day (yikes). I also get kind of ill or grouchy, when I miss more than a day w/o it. Like I didn't have any yesterday due to a mix up between the pharm and I. Yet, I don't think that I would have any problems stopping it, if I were to have to. Good luck, and I hope it helps you
Jack