chronic fatigue syndrome
"I'm finishing the second year of my degree (in neuroscience, ironically), only possible because of this medication. I would recommend that anyone with Chronic Fatigue Syndrome try this medication (give it a few weeks). 
Two important things:

1) The medicine has an 'inverted U' efficacy. In other words, it doesn't do more the more you take, the strongest effect is in the middle. 'The middle' depends on your body weight, but it's usually about 200 mg. 

2) DO NOT DRINK ALCOHOL WHILE ON THIS MEDICATION. I get migraines the following day if I drink while taking modafinil. Last time I had three beers at a BBQ and had a migraine for three days following. It might only be me but don't risk it."