Smelling Salts
One thing I did not see mentioned here was the potential risks of smelling salts. I use them, but am somewhat aware of the dangers. My 2 cents.
As said by someone else above, you do actually take in more oxygen when using this stuff. However, (from what a P.A. I've trained with told me) it actually lowers your blood pressure quite suddenly, which can cause negative affects on your heart. The effect of which has been known to cause cardiac arrest. People have dropped dead on the spot!
The mental side is the dependency and tolerance you can build up. The more you use it and make noticeable improvements, the more you want to use it.The more you use, the more it takes. After a while it can almost become part of your routine and you will not be able to get a good session without it. Obviously, based on what I wrote above this also adds to health risks.
Consult a doctor about this before using. Also, its at your own risk. Stuff happens, no one to blame.
My best results come from occasional use, at my heaviest sets. I use more than I should and am trying to cut it back. The plus - It can take down mental barriers and get you focused/pumped fast! Just be careful.