Cop Out and When in Rome are currently vying for the award for worst overall production of a movie in 2010. It saddens me to say that Cop Out is currently ahead by a small margin.
The difference between the quality of the two lies mostly in the level of talent, and therefore, expectation. I didn't expect much from When in Rome; it's cast and crew were all second tier to begin with and 9 times out of 10, a romantic comedy is going to be pretty awful. The quality of buddy cop movies are always raised by the level of talent. With Kevin Smith directing Bruce Willis and Tracy Morgan as veteran cops who've worked with each other for nine years, you'd think you couldn't go wrong... well, you'd be wrong.
Morgan plays a slightly more sane versio of his character on 30 Rock and Willis plays Bruce Willis, not interested in making this movie. He's phoning it in, and with good reason. Why exert yourself if there is absolutely nothing you can do to elevate the less than mediocre script you're working from.
I'm happy to say that Smith only directed Cop Out, it was written by Robb and Mark Cullen who've dabbled in television, but are highly inexperienced. It's become abundantly clear that Smith's real talent is in the telling of the story (see Chasing Amy, Dogma, and An Evening with Kevin Smith) and not in directing. I'd be curious to see what would come of a Kevin Smith movie in the hands of a good director.
Cop Out delivers a few laughs, but not enough to recommend.