I read many of the other review before adding my comments, so the net result may be a review of the reviews more than commentary on the movie itself. What a twisted, postmodern web I weave, huh?
The first point I would like to address is criticism of the plot - sure it meanders a bit, but it seems more cohesive and lucid than the lives of the stoners this movie illustrates. Also, there is a pretty clear parallel between the stoner sensibilities of the characters and the way the movie was directed. Without giving too much away, I can say that the movie is about inertia. And sometimes the movie is similarly inert. To me this does not challege the the viewer's imagination too much.
There are far more challenging movies out there, even in the same genre (if there is in fact a stoner/slacker genre) - well, Slackers is a good example. It has virtually no plot, no Jack Black comedy payoff, and yet it is a good film. Bongwater, although it does not rely on the same devices, is a good film for the same reasons.
The writing seems to be perfectly good. But there is no accounting for taste. It was not sublime, but neither was it conspicuously bad. I think the characters are rendered well too. Even the smaller parts played by Andy Dick and Jeremy Sisto have a proper amount of complexity to them.
I'll sum up by saying that I think the disappointment I have read here is a reflection of expectations of the movie and the movie not aligning to some viewers' satisfaction. So if you have not read the book, or you have not seen the trailers, and you don't have unrealistically high expectations hot chicks in daisy dukes, prepare to be amused. Excepting most of the scenes Brittany Murphy is in.
I give it a solid B+