I never really knew people to be pot smokers. I didn't go to parties in high school or college, or hang out with my peers much at all. So while I'm sure I knew a lot of people who smoked pot, the first person I knew as a heavy pot user was a guy I knew online who, upon starting pot use, became a stereotypical stoner. He'd been occasionally annoying, but a reasonably interesting person, but after starting using pot, that pretty much was the only thing he wanted to talk about, plus the occasional conspiracy/rant. (As I said, I only knew him online, but I had a friend who knew him well in real life and she confirms my perceptions.)
Later, though, I moved into a student co-op at UT-Austin. Suddenly I'm hanging out with college students outside of class and a number of them smoked pot frequently--some every day. Completely different results! These people were very smart, responsible, academically successful students, mostly upper-division and several of them in grad school. They didn't go to class high (except maybe the theatre grad student, but that's art), they didn't sit around staring at their fingers (usually it was hard to tell that they were high), and they had aspirations beyond smoking pot. And they're not just thinking about their aspirations. Pot smokers I knew who have graduated went on to work for a law firm (and then came back for grad school), went to grad school for engineering a Berkeley (despite job offers from an engineering firm where he had interned), or work in their field. They're useful, contributing members of society who happen to smoke pot in their off time.
So, yes, my perception of pot smokers has shifted from 'kind of dumb' (when I didn't know any people as pot smokers) to 'pot user often equals stoner = useless person' to 'not significantly more likely to be useless than anyone else'.
(Of course, my sample population is biased. There are co-ops where the culture was very much one of useless stoners. I got one where the culture favored being motivated, responsible, hard-working, and intelligent.)