I typically only take it:
a) Have to write a lot of boring code that has already been mapped out, ie I'm not solving any ridiculous problems that require imagination and/or being clever.
b) Learning a new technology I'm not entirely interested in.
c) Learning Japanese
d) Playing poker. I'm a little iffy on this one though. Without Provigil I can play a good 12 hour session no sweat. I have a really shitty short term memory so most of my game consists of the social aspects and the math aspects and reading people, which I'm very good at. I have a general impression of how people play, but I couldn't tell you about a hand I played 30 minutes ago or what anybody did in the hand exactly. On Provigil, however, the game becomes absolutely crystal clear and I can tell you what hand you played 4 hours ago or 4 minutes ago in striking clarity. Really creative playing goes out the window though and I become a very ABC player with the advantage of total recall. I've never had a losing session on Provigil, but I've also never 4x or 5x buyin session on it either (making 4x or 5x my initial buyin) which is probably due to the fact that I play smaller pots because I'm not taking as many risks.
I can stop taking it for months, go on a "binge" and then stop taking it again. It's a lot like marijuana in that regard. I've never mixed the too, btw.