IMHO they are great bikes. I would buy another if I could find one. I owned an '80 GS750E - I think the main diff from a '78 is that the '80 had shim adjustable cams - and it was very fast, very reliable, and made an extremely satisfying sound at high rpms. I took it over 100mph just about every day I owned it. On second thought, perhaps you shouldn't get one for your son.
I looked at a '78 GS750 last week in somebody's front yard. Unfortunately it was rough, lacked the stock headlamp assembly (it had one of the old Vetter fairings on it), and wasn't worth the $650 asking price.