All code review tools sucks hard. They either are so full of themselves they need to take on the role of being your central code repository too (which come to think of it is a fucking travesty we even need in this world of DVCS). Or if they can't pull that off they insist on indexing your repository every minute to be even remotely usable.
Some make it pain to see changes made during the review process (and don't you dare rebase) and other tools hide the individual patches and just show you a big crappy blob. And creating a good system for viewing the comments also seems to be the hardest thing ever. Want to leave a comment on something other than exactly one line? though luck. And to find what have been said you have to shift through all diffs.
Some tools, do some of the things decently but they all fail horribly in some way. I think we might all be happier just using pastebin and email, or just email.
/rant