I had a Grandmother for just over a year. Lots of fun because of its simplicity and directness, and a very pleasing bright and potent sound quality. But like MrSmith I have found the Pro 3 to be at least a match in terms of tone, and it's obviously massively more sophisticated in nearly all other respects. The GM was gratefully snapped up by a student. The main issue I saw with it was very poor control of short envelope attack times - it squeezed that crucial 0-50ms range into about the first 1mm of travel. On most synths (with digital control) the same is represented by a 1/3 or 1/2 turn of the pot. Several other units I tried in retailers were exactly the same. In the end I sent it for a custom mod at the Moog UK service centre, but they could not find a pot replacement or modification that gave more control over fast attacks without adversely affecting other aspects of timing or slope, and we abandoned the idea.
Currently I'm working through 3 banks of a SUB37 that I used for a live project, recreating patches on the Pro 3, before hopefully passing on that Moog too. Direct recreations are proving a little harder here because of the distinctive dark/rough SUB37 tone. Strangely the Pro 3's 12dB SVF often gets closer to the character, even if the SUB37 patch was in 24dB/oct mode.