Maybe already today Zemanta will reach an important milestone of 1000th code review! Our first code review was posted on February 28th, 2011, which means we post on average 4 code reviews per day. When we have introduced code reviews, we have said that it is up to a programmer to decide whether to send a code for review or not, but now we review all code changes. In that way the programmer doesn't need to ponder whether the code should be reviewed or not. He justs posts the code for review. If the change is trivial, the reviewer won't spend much time on checking it, and if the change is complex, it should be reviewed anyway. Additionally, there are no trivial changes. The shortest code review where a bug in my code surfaced consisted of just 2 lines of code! Code reviews are not panacea and won't resolve all your problems. But it's perhaps the most effective team building practice available. If you don't do code reviews, you should start doing them immediately. And if your team members show resistance to introduction of code reviews, you should seriously ask yourself what is the underlying cause for their opposition. Are the people on your team ashamed of their code, are they protecting their walled gardens, are they having poor opinion of their coworkers, or they just don't understand that software development is a team effort.
For some great guidelines regarding code reviews check code reviews at Khan academy.
UPDATE: The winner of the 1000th code review contest is Tomaž Kovačič!