I had one for about two years when they were available. Very accurate gun...I could easily roll 8in. head plates offhand at 100yd. with it using good ammo. Downside is it is big...I mean, BIG. It also heats the barrel up a lot after a box or two of ammo...so hot it will burn through vinyl seats if laid down on them with slide locked back
I have no idea of their value now, but they never sold very well so I suspect there are not all that many around. I would think $650 is very high for a used one around here...$450 would be more like it if in really good condition. (And, BTW, I would think you would be pretty much up the creek on extra mags for it now as well...the mags do not interchange with anything else I know of)
I think you could do much better for an investment at that price...but then I'm no collector so WTF do I know?