I’ve never used QW. I did try MobClix and have flirted with Greystripe (though never implemented it).
GreyStripe: Their ads (large “premium” ads that - at least when I last looked - had pretty long loading times) simply don’t fit my apps. I would consider them if I ever did have an app where I think my users would accept staring at a full-screen ad for any length of time, but for my games… no.
MobClix: Very permission-heavy library. They improved this somewhat in recent versions, but it’s still too much IMO. In addition, they have a reputation as bad payers, and given that I’ve never seen a cent of money from them, I can only agree with that assessment.
AdMob: They sometimes have problems with low impression rates, but high volume apps seem to get access to Google AdSense ads (P&T does), which helps a lot with that problem. Not missed a payment yet for me. At the moment, I don’t use anyone else - I really don’t see the point.