And they don't tell you not to use it with heroin and cocaine either, so that's what I do. (joke)
I just posted a reply to the 'bent willy' thread but to elaborate, in my experience it starts working after about 30-60 minutes and basically it exacerbates the state you're in, so if you're feeling slightly horny you'll have a massive stonker. I discovered this to my cost in a fortunately dimly lit nightclub. The effects seem to vary a bit too. On one occasion I took one pill at about midday (pre-shag) and the effects were still noticable for the evening shag (this being something of a rare 'twice in one day' experience for me, in case you think I'm bragging). On another occasion the effects started to wear off after less than 1 hour. Just as well really, because poor little Fido was getting quite tired!