I've generally found pair programming to be more useful than code reviews. It's too hard to drop in at the end of someones weeks of work and try to meaningfully review it. And a lot of suggestions you might have would have been useful at the start of the work but aren't worth going back to fix.

If you actually sit along and see the whole process you can actually understand the whole piece of work as well as give suggestions when they are useful and much cheaper to action.