I worked at a State University for five years, and they used the "2 letter, 4 number" hexidecimal-type system to issue ID numbers to all faculty, staff and students. An individual's University email address was the same as their ID number.
I didn't care to handle personal emails on my work account, so I opened a Yahoo email account for personal use. As I stared "like a deer into the headlights" at the signup page, I realized that I wasn't feeling creative enough to come up with a really catchy user name, so I decided to use the "af7850" that I was issued until I came up with something I really liked.
I don't know if it's because I'm lazy, picky or just don't really care too much, but it's been 10 years, and I still haven't ever changed it.
I think, though, that its kinda nice to not be pigeonholed as "harleyguy" or "Iammarriedwith2.5kids", etc. Maybe I'm just making excuses, but I've grown to like having a non-descriptive username, as I think it allows people to not judge me until they read what I have to say.
But then again, maybe some people read "af7850" and think, "what a jacka$$." Who knows?
