My neighbor here has a 1978 (ish - I forgot) BMW r100.  It's an ugly brute, but in decent shape and good running order.  Any thoughts?

A bike of somewhat legend. If you want a steady ride that's one. Far better than Moto Guzzi and I know that for some that's not saying much. Depends on how much he's asking.

I think around $3000.  I'll try to get some pics and better details this weekend.  It's not a show bike, that's for sure.

Have you ridden it?

The torque reaction on the bike from blipping the throttle and the shaft drive jacking the rear suspension up and down do annoy some people.
Ginormous flywheel and clunky shifting also annoy folks.
Generally they have longer travel suspension as well, so there's a lot more motion than folks are used to.

I spent a good bit of time on an R60/5, wasn't bad but didn't speak to me.

Not yet.  Neighbor just decided to sell it after a few years of it sitting and getting started occasionally. 

I have a little experience with shaft drive, but none with this one in particular.  If we get warmer than 40 or so this weekend I'll give it a go.