I'm not a big fan of shower sex, but it's only partially because of the sex itself. The simple fact is that I'm too damned tall, and the problem is exacerbated by the fact that I have a thing for smaller girls. The combination of all three just isn't meant to be. The only way I can manage to do anything at all is by either holding the girl up the whole time or semi-squatting in some bizarre position; it's just not fun. And I can't get very rough-- not that most girls let me for long anyway, but I take what I can get --because I don't have leverage. Eh. The heat & steam complicates things too: once you start getting into it, you really start to notice the lack of good oxygen supply. On one hand, it's kind've fun, in a kinky sort of way, but not having a good oxygen supply tends to suck after awhile. On more than a couple occasions, I've finished up a girl in the shower, laid down to catch my breath, and completely passed out-- for more than an hour.
ASIDE from all of that, shower sex can be great. It's definitely worth trying, at least once. Hell, I think most things (sexually, at least) are worth trying at least once. Sometimes you can get your girl to be a little more dirty than she's usually up for, because she's in the shower and everything rinses off. And I have to admit, something about a girl just grabbing your cock while you're taking a shower and then going to town on it is supremely hot. I've taken quite a few showers with gf's/friends, and most of the time the girls act like you're not even naked. It's pretty fuckin nice when you're just doing your thing, rinsing off or whatever, and you feel a hand grip your cock and suddenly a normally monotonous experience becomes very, very heated.