I bought a bunch of modafinil tabs, only used it once so far. Took 200mg, was gonna spend most of the day studying and catching up on homework for school. It didn't turn out too well, although I was extremely alert and energetic, I had extreme difficulty concentrating and keeping my focus. I found that I would read stuff and not recall what I just went over, I literally had to tell myself to slow down and read sentence by sentence and then let it sink in.
Also had serious issues doing high level analytical problems (math major), so I'd have to agree with the above poster, it was really felt like my problem solving skills and logic process was incapacitated, or at least slowed to a large degree.
Perhaps 200mg was too much, I'm not through experimenting with it, but I was very disappointed with my first try.