Too broad a subject to generalize. Just as is true with any genre, there are good ones, and there are not so good ones. To be frank, I didn't really enjoy the two movies you mentioned.
I think "Half Baked" is pretty funny. So is the first "Harold & Kumar". "Up in Smoke" is classic. "Friday" is pretty damn good.
With those few exceptions, though, I think it might be a mistake to make a Stoner Comedy, wherein the entire movie is about smoking pot. In my opinion, I think this type of humor gets more mileage if it's not a Stoner Comedy, but has elements of Stoner Comedy in it. Like "The Big Lebowski".