I bought my SY-85 in late 1992 and got the impression they came out in 1981. I've used and abused it for most of a decade, and have come to the conclusion it might be the best sample synth ever made. It does, however, have some drawbacks. First, it ain't General MIDI, mostly because the spec was too new when it was released. Also, Yamaha never released additional patches for it on cartridge as they promised, so the only place you can get samples is on 720K (yeah... gee, they save $4 per unit by not using a 1.44) diskettes. Its support for SDS dumps isn't great. The sequencer internally isn't all that great. Some of the menus are a little inscrutable and the user manual isn't always helpful. However, it sounds tremendous, feels good to play, is durable as anything and you can still get parts and service.
I'd say $575 is a decent deal if it all works. Make sure it comes with the 64K RAM cartridge. New, it was $1800.
Turtle