At the time I post this, 80% of people have responded that they do not have HPPD - which honestly doesn't surprise me. It also doesn't surprise me that the rate of HPPD here is so much higher relative to that of the general population; this is, after all, the "Psychedelic Drugs" forum of a chemical-based community

%)
I responded 'yes' to the poll, because I do have HPPD, but over the past several years the symptoms have markedly decreased. This leads me to posit that HPPD may not be entirely permanent, or may even be reversible (either of these could already have been proven to be true, I haven't really given those specific issues thought until thirty seconds ago! Forgive me!). Interesting areas for future research, though.
I believe I first developed HPPD the year that I joined Bluelight (which would have been 2003?). In 2006 I succumbed to the darkest depths of addiction and consequently spent the next two years completely sober from all mind- or mood-altering substances. During this period of abstinence from better living through chemistry (after an active 5-year stint as a chronic, daily polysubstance user), the HPPD ssssslllllooooowwwwllllyy began to diminish in what I'd call 'intensity.' It has now been six years since I first experienced HPPD and I would say the symptoms have decreased by roughly 10%, but at their lowest it was probably reduced by closer to 25%. It didn't take very long after going through a heavy period of use earlier this year for most of the symptoms of HPPD to re-flood my visual field.
Much in the same way some people come to see and recognize those "floaters" in the lens of their eye so often during life that they come to like them, so too have I developed a certain "relationship" with my 'modified' vision over the years. I think that I'd be a little upset (and a LOT more bored!) if all those cars' taillights stopped transfiguring into snakes and comets at night on the highway 8(
Anyway, I'd better get off this computer before the movement of the text begins to get to me :D lol
~ vaya
edit: just read the other replies. Interesting discussion so far. One last comment from me regarding the 'legitimacy' of HPPD: I can understand people's general skepticism. It can be daunting to consider the implications for someone who is literally 'tripping' all the time in a purely visual sense. But as someone else mentioned very wisely, those of us who DO have it, KNOW we have it.
I have only met one other person face-to-face in my ventures who shares this affliction. That lends support to the suggestion that was made about its infrequency in the general population.