i beat GTAIV a half a dozen times... i only beat SR2 and 3 once each... with both i started a second or third file, but never went far with them... IV was go engaging, i enjoyed each and every playthrough... i even beat the episodes a couple of times...
like DiO said, the side stuff is fun, but once you grind them all out for 100%, you don't really feel like playing them again... but i could seriously throw in GTAIV right now and play it all over again...