Code review takes the form of pair programming. That is, two programmers sitting side-by-side, one driving and the other paying close attention to the task at hand. So, it's continuous code review.
Even thinking about such a practice gets me angry. It is treating junior programmers like dirt. When the time comes to go for the next job, what is the junior programmer going to say? "I never programmed a line of code, I merely examined someone else's?"
And how is the junior programmer going to learn to recognize good code if he merely looks at someone else's and never writes code himself? It's like the line from the Gilbert and Sullivan song:
Stay by your desks and never go to sea
and you all may be the leaders of the Queen's Navy
Moreover, what senior programmer wants someone else looking over his shoulder 40 hours a week? Often I try something and it doesn't work. Last week I spent several days trying to get user_manage to work before giving up and using Apache::htgroup and Apache::htpasswd instead. Think how embarassed I would have been to tell a junior programmer that all the work he did supporting me was for naught, we are not going to use user_manage after all.
I don't know much about extreme programming, but just hearing about it upsets me. Save "extreme" for extreme sports and tattooed skateboarders. Let's develop software using moderation, not extremism.