Boogie Nights" and "Goodfellas" lite I've heard this being called and for the most part its an apt description. Like those films this is the story of the rise and downfall of a man involved in some rather questionable business - unlike those however this takes a lighter tone in its analysis.
There seems to be two major reasons why people don't like this - the first is that it's a clone of the aforementioned films which are, from a filmmaking perspective, certainly better movies. The other is the questionable morality that comes with the fact it makes you sympathise with one of America's biggest drug dealers - neither of those factors bothered me so I really quite enjoyed it.
Demme has created a solid and interesting character drama which draws one of the best performances out of Depp in a while - after the laughable "Sleepy Hollow", its good to see him finally tackle a role with a bit more substance. Penelope Cruz, I'm sorry I just don't see what it is about her that has made her the "hot thing in Hollywood" and her bitchy role here isn't performed well either.
Faring better in about 1/3 the screen time is "Run Lola Run" gal Franka Portente as the stewardess Depp falls for, and Paul Reubens steals every scene he's in with the part of the flamboyant Derek Foreal. Rachel Griffiths and Ray Liotta are really great as Depp's mother and father, and the list continues. From an acting standpoint, Cruz is the only weak link amongt the bunch.
The style of the movie is also very authentic with a killer soundtrack, wardrobe and some effective makeup that ages Depp in different periods (check out the frighteningly authentic mid-40's beer gut in some scenes). Then there's the story itself, which is where the faults do lie. Does it glorify drugs? In the first act it most certainly does.
If you leave at the 30-minute mark you'll want to break right into the world of trafficking. Like all these tales of course, Depp's character goes downhill and thus one begins to see it as a little hazardous. But whereas "Boogie Nights" turned dark and tragic to such a level it really did "put the fear in you", "Blow" never really gets that dark - the ending is sad, but throughout the movie it rarely makes moral judgements about the actions of its characters (much like most of "Traffic").
Its a solid film, certainly a lot better than the crap out there this month, but it certainly isn't anything new - deriving bits and pieces from all sorts of previous films in the genre without establishing a style of its own. You won't find any insight to the drug trade here but you will find a decent movie.