I find most films really don't get it right. It's far more rare to get an accurate depiction.

The first realistic scene of what it's like to be a pothead is when Seth Rogen goes to pick up from James Franco at the start of Pineapple Express and they smoke the cross joint together. I've had dealers that are literally describing their bud with the same mannerisms as James Franco has during that scene.

The starting sesh in Half Baked is pretty spot on too, even down to the random dude who's too stoned sleeping on the sofa in the background.

And while it's a tv show and not a movie, American Dad has a realistic depiction of being stoned when Stan gets baked and spends 5 minutes in the store with red eyes trying to decide what chips to buy.