Well, some cons...
(Depending on sativa or indica) it makes me sleepy.
It makes me hungry.
It makes me want to... er... I forget. Oh yeah, clouds!
AT FIRST, it makes you a little paranoid. That could also be put down to the fact that its illegal though - when you first started at 15 or whatever, didnt you keep looking around for cops and parents? When you get older, you adopt a 'I dont give a shit, I know the truth about it now and its my body, my life' attitude. Or maybe thats just me

Some pros...
(Depending on sativa or indica) it makes me energetic.
It doesnt give me the munch anymore... unless I start eating.
It makes me forget my troubles IF I WANT TO
If helps me look at my problems from a different point of view, thereby helping me sort them out more efficiently IF I WANT TO
It makes me interested in even the most mundane of tasks.
It helps me sleep at night (my sleep pattern is shite, probably sue to the fact that I seem to be suffering from insomnia lately).
It makes me more relaxed and chilled out, just taking things one step at a time.
Food tastes so much better. My smell, sight, sound and taste is enhanced tenfold when Ive been smoking some Herb.
Its a social thing to sit round a lava lamp and pass a spliff.
Its far safer, and cheaper, than alcohol. Also it leaves no nasty hangover in the morning, and you dont wake up with a sleazy whore next to you the next morning.