It's pretty embarrassing now, but I have a lot of sex stories from when I used to use amphetamines, and I thought it was the worst drug to have sex on. But I do have one qualification that I'll mention at the end. (Also, I could talk about weed and alcohol, both of which I haven't had positive sex experiences on, but I think my stories about having sex on amphetamines are more interesting and open-ended).
So, one time I was on some Vyvanse and I had just met a girl at a club - the club closed and we went to her apartment - we made out and built up to sex for about 30 min. and then we went into her bedroom to get it on. Haha, I actually remember telling her on the car ride over, because drugs came up, "Oh yea, I used to do drugs, but I don't do that shit anymore" - anyway, long story short, it took me an hour and a half to even get hard enough to fuck her - I have no idea how she was so patient. But when I finally got it up, I fucked her for a LONG time, about 2-3 hours, not even kidding. It was some of the best sex I've had. The only problem was that I couldn't cum into her - after a few hours, she was exhausted as shit, I'd been pounding her like a motherfucker, and we just passed out.
Another story is with the same girl - I ended up staying at her apartment for a few days and one time we got some Wendy's and we were chilling on the couch - she was horny and she wanted me to fuck her, but I had just taken an Adderall before we went to get some food. My solution was to fuck her in the ass, because it was easier access and easier to penetrate. The funny thing was by this time I could only get half hard because the Adderall had been kicking in while I was eating, so it was still pretty hard for me to get my dick into her ass but I did and fucked her as hard as I could like that - still couldn't cum this time again, due to the amphetamine side effects, but I fucked the life out of her in about 20 minutes. It was ridiculous - I still remember how we were watching 'King of the Hill' when we were fucking - very surreal shit looking back on it, that happened 2 and a half years ago.
A third story I'll give is simpler - I went over to a chick's place to hookup and was high on Adderall - we proceeded to drink some wine, watch some porn of having a girl double penetrated by dudes, and then she started taking off my pants. For the next hour I got the best blowjob of my life - I was hard as shit the whole time and she looked so fucking hot...but, well, I couldn't cum. I just couldn't. And it got pretty awkward - she questioned my sexuality and all this bullshit about what I really wanted out of a relationship, and I mean, hell, I had just gone over there to get my rocks off and I couldn't even do that because I was fucked up on Adderall (and weed too, in this story, but nothing more than a bowl a few hours before). Anyway, I spent the night, but we didn't fuck and I never saw her again, what a surprise.
The caveat I was waiting to say about Adderall is the same thing the OP mentioned - it's next to impossible to perform on it, I don't care how horny you are and how into it you are. If you can even get a stable erection, you're pretty lucky, in my book. And also, of course, it's not healthy at all - you're not doing your body any favors, esp. if you're the guy who's fucking the girl, by making yourself put out all that energy. I didn't continue this practice of having sex on Adderall/Vyvanse for more than a year.
However, all that being said, I did have some fantastic times on Adderall with sex - even though I couldn't cum, when I was able to fuck girls, I really pounded them to shreds, and they all tried to get back with me. It is what it is. (Further, if it hadn't been for the amphetamines I was taking at the time, I would have very doubtfully ever even gotten together with many of these girls - I was much more courageous on Adderall/Vyvanse, and although I'm naturally good looking, it became ten times easier to approach girls when I was fucked up on amphetamines - I felt and I knew that I could get whoever I wanted, which was a crazy feeling to have. I wish I could just go back to that time in my life and stay there, for real).
I will say this though - I haven't done it, but just from my understanding, it'd be very easy to really enjoy sex on stimulants if you're the bottom and taking the fucking - the problems come in when you're the active top, which I am and was. Oh well, I'm not going to change my orientation or method of sex just to take stimulants at the same time, lol.
Just realized I have easily the longest post of anyone on this thread - hope everyone enjoys my stories, they're all very true and I had a great time doing it. The sex and relationships were easily the best part of my experience using amphetamines, and taking them made me hyper-sexual. I even cheated on my girl a lot during that time, which is something I normally wouldn't think of doing.