I'd say it depends on the person also. I have a mate who gets quite motivated from weed, he has cones all day long and runs his own business flat out. I also have mates who get real fucken lazy and don't want to do anything.
I have a few older mates who have been smoking for years (20-30 years) some are like zombies and some are fine.
Personally myself I have smoked for 18 years everyday (well almost) and it does make me lazy. I work away three weeks at a time, I feel fucken great after my swing, no piss, no pot for three weeks and i'm motivated to go home and get things done, two days back on the choof and I cant be fucked. In fact I am quitting now, I've noticed a big change in my behaviour over the last two years.