It's funny 'cuz it's true...
Kevin Smith started an interesting comedy sub-genre way back when with Clerks. It's the original "My Job Sucks" movie, and its conventions have been followed faithfully at least a couple of times since. Basically, it's the low-budget movie about a specific type of job and the poor saps who have to do them. They're never really great movies, strictly speaking, but they're generally pretty funny and wind up especially beloved by those who labor in the industry being lampooned. The most popular (and probably the best) movie out of this mini-genre is Office Space.
A newer addition is Waiting..., which Mle and I watched last night. It's not really a great movie, but it's pretty funny and is certain to wind up a particular favorite of people in the restaurant industry - especially the shitty restaurant industry. It's kinda hit-and-miss, but it really rings true in a lot of areas.
I will say that, in my experience, the "kitchen/service staff will spit on/pee in/wipe their asses with/otherwise do disgusting things to the food of unpleasant customers" thing is one of the great myths of the restaurant-going public. I've never done such a thing, and I've never seen it happen. But the strange and quite often disgusting running jokes and weirdness happen (though I've never seen anything quite so extreme as the Penis-Showing Game), the staff bitching about customers in the back and then coming out to the front all smiles, the constant near-incestuous hooking up with other servers, the passive-aggressive comraderie and after-hours boozing, the smarmy middle-aged manager flirting awkwardly with the high-school-age hostess, the nearly-as-smarmy mid-'20s server who's actually hooking up with the high-school-age hostess...well, "it's funny 'cuz it's true."
I'm not really wild about Ryan Reynolds, but his schtick is well-applied in this role. Luis Guzman is good as usual. And how bad can any movie be that gives a small-but-crucial role to the great Sam Weir?