Modafinil increased my blitz chess rating by 150 points the first time I took it. That is completely ridiculous. The effects weren't as strong after that, but still very noticeable. It definitely worked to keep me awake for long hours. One time it totally fucked me up for 3 days or so: I took it thinking I'd stay up, but then I was like nah, never mind, I'll go to sleep 'cuz I'm really tired. Like an idiot. I then got 5 or so hours of fitful sleep followed by about 5 more hours of thrashing about on my mattress follow by about 10 more hours of groggy sleep deprivation-induced pain followed by a completely messed up sleep cycle for the next few days of constant tiredness. So, it's awesome when you use it right, but be careful to stay up for the duration of the effects (12 hours or so, I think.)

I am quite obviously not a doctor, though.