I've seen evidence of both. I've known some big-time potheads who ended up (1) owning their own company after being president of another or just plain out (2) succeeded in a decent career, grew up and matured and also (3) ended up being a bicycle messenger and at age 51 that is still what he is doing. He still looks and sounds like a total burn-out too.
I think it depends on a lot of factors but I won't dispute that the likelihood to be #3 in the above scenario is greater with abuse of pot.