I actually have GEOS but I never used it. I snapped up a few Commodores like two years ago and one really had a ton of stuff with it. Most interesting for me were the cartridges that could be used for freezing and reverse engineering but it also had GEOS and some kind of drawing tablet.

Watch out when you buy them though....the transformers are notoriously bad and could fry your SID chip pretty quickly! Always warm them up for a while and measure the lines before connecting it to the C64!