I have an aftermarket gasket set for my CB350twin, and even when the head gasket is there, I think I will err on the safe side and buy the OEM honda. The whole set costed me about 25 bucks + shipping. If i'm not wrong, the head gasket is the only one exposed to high compression and temperature, so it is important that it is made of the best quality. The othere ones simply have to keep the oil inside, so I guess it's not that of a problem. Also, except the cylinder gasket, almost the rest of them (at least in the twin) can be replaced with the engine on the bike, so if they fail I can always replace them later for a OEM gasket.
Raul