Amphetamine is still prescribed in some very specific and very rare cases of obesity (Adderall and dexedrine). Basically only if other things don't work and the person is physically incapable of exercising alot, and not yet having developed heart problems or high blood pressure.
Are you sure about the impotence claim? People take amphetamines before sex all the time, and while dopaminergic stimulants do cause..um..shrinkage worse than icewater, erections and orgasms are still not difficult. Ecstasy wouldn't be so popular otherwise.
And insanity, definately not unless you're taking way too much for way too long. I think you forgot Requiem for a Dream is portraying a worst case scenario from extreme amounts instead of patients following the directions.
Moving along, mild legal stimulants will help you lose weight by appetite reduction. But with these, you still have to make an effort to control your eating, you can't expect them to do all the work alone like strong stimulants. Before attempting to use a stimulant regiment for a diet, it's highly adviseable to have a full workup done to check for heart problems, especially if you have a family history. What I believe to be the best stimulant diet aid program that's legal is caffeine and ephedrine. Ephedrine was banned from being sold as a diet aid because people with heart problems were having them made worse, hence my advice to get an angiogram, but it's still legal to take, and can be bought as a decongestant (Sudafed, anything with either ephedrine or psuedoephedrine (I believe they're just optical isomers). Keep the dosages of each as low as you can to get your appetite under control.
Interesting side note on ephedrine, the only difference between ephedrine and methamphetamine is one hydroxyl group attach to a carbon atom along the side chain from an aromatic ring.
DISCLAIMER: I am not a medical doctor, consult one before using any substance as a diet aid.