In the better case, they just become insufferable and pseudo-intellectual because they started watching Alan Watts and Carl Sagan while stoned and would become convinced that they know everything about physics and philosophy.
In a lot of cases though, and this is more obvious in hindsight, it feels like they were using weed as a means of dealing with the fact that they were deeply unhappy and depressed people. Instead of confronting their problems and seeing a therapist/psychiatrist or any of the other things that they could do to actively improve their life, they would spend their evenings and weekends getting high.
I don't inherently have an issue with people using recreational drugs; I've gotten drunk before [1], but it should be done in moderation.
[1] I never did it that much and I haven't had anything to drink at all in years.