I'd say that's not a bad price, but not a bargain. I think average is about $650 on armslist with occasional chances under $600. If its the gun you want, I wouldn't feel bad about paying it.
It's not a great price. But you're not getting bent over either. I paid less for my 5.25 brand new. Pre-panic price was $540 for the 3.8" .45 version in black, and tack on $50 if you want the stainless 2 tone version.
I wouldn't pay $635 + ffl transfer fees for one, put it that way. But if you don't care about $120 ish, then go for it.