Very fast and easy to use as standalone or as editor's plugin.
I found WinMerge slow. Running from Server 2008R2, trying to compare a local 60GB tree with the same tree on a server 2003 on the network. Been running an hour; only about 1/3 of the way through. Robocopy compared them in 5 minutes.
Great tool, a bit bulky, resource consuming and slow, those are its weak points, but it does the job fine.