I have had a bit of experience using different code review tools: Gerrit most recently, and Atlassian's FishEye in the roles preceding that.
Strangely, the most effective code reviews that I have participated happened at Sophos, where no tool at all was used: The rule was simply to get another developer to sit next to you when you did a commit, so you could talk him through the code line by line before submitting the change.
The experience there was that most (if not all) of the problems were spotted by the original developer, not by the reviewer, who frequently lacked the contextual awareness to identify anything deeper and more significant than the usual picayune: formatting and layout errors -- the real value came from being forced to re-examine and explain your work - controlling the searchlight of your attention into a more disciplined and fine-grained search pattern than is possible alone.
The other types of review to which I have been party:- asynchronous, distributed reviews mediated by a web tool of some sort, as well as formal half-dozen-people-in-a-room-with-slides style reviews have, in my experience, proven far less effective.
So, I sit here wondering if we can rescue the asynchronous distributed code review tool, either through an alternative approach or the application of a formal and disciplined approach of some sort ... or if it is doomed to more-than-uselessness?