I have used my SQ-Ai now 8 years (before that it was used 6 years by previous owner).
Electrics on the body, lenses, AE-finder, TTL, motorized grip and film backs works fine except two of my five film backs that has recently begun to show some problem with connection to AE-finder.
As film speed information is read by AE-finder via simply contact pins, i guess that either those pins have worn somehow or these two backs are simply used so much that they do not fit stiff enough. The AE metering works if i push film back bit upwards.
The film speed is determined by resistance, so the electrics on the backs are quite simple.
Otherwise the biggest issue is batteries like already mentioned. However when using motorized winder grip, that is not a problem.
The winder will 'go slow' before voltage drops enough to affect shutter.