W.O.E. has some other great posts as well, if you're into that kind of thing, and a good instagram account.
From the Casio article:
"The Casio F-91W’s reputation looms large in both horology and national security circles, and for good reason. The simple, cheap and effective plastic watch is likely one of the most ubiquitous timepieces on the planet, with an estimated three million produced each year since sometime in the early 1990s. However, the watch that is coveted by hipsters and former presidents alike has a more sinister utility..."
I've had a F91 in the past, but the band broke and I haven't had one since. Maybe I'll pick one up in the future if I get a good tip or find 20 bucks on the sidewalk or something. There's a Casio that's a bit more updated, with similar specs and a way better light, but without the adulation and infamy of the F-91, but of course a good chunk of buying a watch is its reputation.