These were common and fairly popular in the late 70's early 80's. They are useable. Not really special from what I remember but back then there wasn't a lot of other choices either.

we've been using ours on drums (toms and kick) with good results. the expander is a nice feature. haven't tried it on vocals though.

I have one, I find it a fairly agressive and fast compressor. Great for Big Spanking.

I picked it up for $50 from a radio station.

I've just brought one home to try out.. The 20uS attack setting is fun.