It is well know that code review is very effective in reducing bugs and improving code quality. Be it agile, when two developer are working over the same code, where review is happening while it is being coded, or when it is done by peer once a developer has finished with. Both has positive effect on the code.
But there are other trick issues involved with it some organizations.
You many want to look at this
http://smartbearsoftware.com/codecollab-book-review.php There are two more books good books on this, i will try to get this information and post it later.
Thanks,
Vijay Venkataraman