Funny, in a very crude way. But it's also very sentimental. Rent, if you like Kevin Smiths' films- or 80s films in general, because it seems to be inspired by films from that era.
Aside from the fact 'Zack and Miri' is pretty much a shameless rip-off of the 2005 film 'The Moguls', this is just a really bad film to boot. It's hard to believe this is written and directed by the same guy that gave us 'Clerks', 'Chasing Amy' and 'Dogma' as this pure garbage aimed at the 'Harold and Kumar' demographic. If you want a genuinely warm and likeable film which follows a similar storyline then rent Michael Traeger's 'Moguls'.
The premise from Smith is interesting, however as usual, his self referencing style soon gets the better of the script. The sentimental aspect interferes with the overall comedy, soon derailing the filming. Zack & Miri does continue some gutter humour and full exposure, but in a harmless way. It easily slots in the slacker comedy genre but lacks laughs.