I've been using 100mg on days where I want to get a lot done, or where I want to read (stuff that needs proper focus) at 7:00am on the train (am studying).
I found the 200mg pills would get in the way of my sleep patterns, likewise using on consecutive days.
I also drink less caffeine on days when using Modafinil too as it creates the same effect as drinking a dozen shots of espresso.
I get it online in the UK using one of the many online pharmacies - never had an issue.
I used it today - was up an hour earlier than my normal work day, I worked solid on my hour overground commute, led a 7 hour workshop and followed that up by running a 2hr client meeting.
I'm also a daily weed smoker and it does a great job at clearing any brain fog.