I took half a pill a few years ago, when I was 29.
It was kind of fun, but I don't think I'll do it again, at least until I'm old, and even if then, only if I need it.
I got an extremely hard boner that just wouldn't quit. My girlfriend loved it. We basically screwed for hours, but then, eventually, both of us sort of got, well, tired. But guess what? That boner was still there, throbbing away (and so was my head). The sex was pretty pleasant, but the sensation in my cock was less than usual, and it took me forever to cum.
But then, I usually go longer than the norm before I cum, so it may just have been how the stuff interacted with my system.
So, I guess I might recommend it for a young guy who's about to make it with a hot woman (or guy) whom he really wants to impress. But he shouldn't count on the experience being all that mind-blowing for HIM. (Unless, that is, he has a stroke, which i understand has been known to happen as a Viagra side-effect ... talk about mind-blowing...)
Finally, what I'd REALLY like to experience is having sex with a WOMAN who's taken Viagra. I've heard that, for the guy at least, its effect on women is amazing. Anyone ever experienced that?