On the receiving end, it's varied between girlfriends.
I've always found myself enjoying sort of dominating while my girlfriend does it, but I've never been given the fortune of finding someone without a gag reflex... so.
I enjoy the shit out of them, but it's not my favorite thing in the world. There are other things I'd rather do, but it's not like I'm going to say no.
Personally I enjoy reciprocating, it's fun as hell.
I have had girls with gags and without. I can go either way really, they both have their pros and cons.