Great thread - I'm a bottom, so it's fascinating to get tops' perspective on this. I kind of feel bad sometimes, because I oftentimes don't cum-cum when I'm fucked (prostate orgasms are a different story), but its really more physiological than anything else - it just takes me a while to orgasm, largely because of the intensity of the pleasure I feel when I'm being fucked. But whether or not I cum has no bearing on whether I'm enjoying it, or getting off on it. I don't think any guy who's fucked me has ever gotten the impression I didn't love it, quite the opposite. I'd just hate to for a guy to think I didn't simply because I didn't shoot.
Mind you when I do it's goddamn off-the-charts LOL.