This movie is just a lot of fun
It literally starts off with a talking joint giving you the instructions of how to watch the movie. It makes the comparison of watching this movie sober to watching a 3D movie without the glasses; you need to be high. And in a way it does make sense that reviewing this movie after a sober watch wouldn't be doing it justice, since that wasn't what it's intended for either.
So right off the bat you know the movie isn't taking itself too seriously, and so shouldn't you. The mere concept of two of the world's most (in)famous stoners as protagonists in a high school-movie is already entertaining, and on top of that the performances weren't too bad either! In a fun way you can see the difference between your average actor, whose acting-classes for "realistic acting" can almost be seen leaking out of their nose, to the point that it's not realistic anymore, with Wiz and Snoop, who are so much the polar opposite to that mentality, thus making them more realistic as characters. I'm making it sound worse than it is actually, as this difference is one of the movies' charms aswell, painting Mac and Devin as the good guys they intend to be.
On top of that, this movie is quite hilarious in its meta humour, of the joint coming in once in a while (and actually even giving useful information). It's got the right amount of laughs, right amount of music (kind of making it feel like a parody of both high-school movies and musicals), and it's self-aware enough to not feel awkward, but also not too self-aware, to the point that it'd become irritating.
All in all this movie is a great time, made by people who know really well what they wanted to make and definitely succeeded. Just don't watch it sober ;)