Most of my G4s have had lovely hair, only 'playful' Fluttershy has horrid, greasy, stringy hair (she's the darker pink version), but they have all needed a good brshing and finger styling and most have needed a little trim to get rid of random straggly bits.
I dislike the sewn-in strips in the hair but they aren't too hard to get out, likewise the sellotape is annoying but manageable. The worst so far has been the CMC set which I sliced my finger open on the jagged plastic tray. It's the first set I've seen with raffia (?) string though, I assume for recycling purposes.
The head/body colour mismatch and the slightly off printing of the eyes are (to me) much worse than the quality of the hair though. Much as I do love G4s the QC issues are really irritating.