I used to take Xenadrine years ago before they changed the formula.
I can tell you that it's effect on me was very, very intense. I've never taken steroids, but I can imagine what a roid rage feels like.
You could feel your heart race, your limbs slightly trembele, and all you wanted to do was work out until complete and utter muscle failure. And in the past, where your muscles would normally fail, while on Xenadrine, you could get another 5 reps in. Or run a little faster and a little longer. My body was doing things I didn't think it could do.
If anyone's seen the movie 28 days later, where the zombie people would just fucking go and go and go until someone killed them, that's what it felt like for me being on Xenadrine. At some points it even felt scary. Like I was some kind of posessed being able to fight the entire world all at once... and win.
I don't know if this is how it was for most people, or if I just had a weird reaction to it... but it was pretty frickin nuts.