In the past I've found kdiff good but not very intuitive when doing more complex merges, and just not as pretty as Beyond Compare. But for example I don't know how the cleverness of their compare algorithms are or how some of their more advanced features compare. What do you get for paying some money?
Mostly I'll be using it for three-way merge on individual local files with TortoiseHg or SourceTree. i.e. not remote/SFTP/etc and not comparing folders.