I bought a GI in expectation of haviong several things changed, including new sights, grip safety (to a beavertail), mainspring housing (to a flat), and possibly some other work.
I've been shooting it for quite some time now, with probably 800 rds through it. Quite frankly there is no impetus to customize, or operational problem to account for, so I may never bother.
Sights work fine. spot on. Ok, they don't glow, so what. Ejection is positive, and they always pop 3ft straight up, and 2ft over, on my right hand side. Nice little pile. I don't reload, and never bother to look at the spent casings.
If you have no plans whatsoever to customize, get a Milspec to be safe. If, for some reason, you can't get a hold on a Mil-spec, and CAN find a GI, just buy it. It's not that big a difference, and is less money.