I've evaluated DeltaWalker, Araxis Merge, FileMerge, Changes and DiffMerge. I've never evaluated Guiffy.

I'm a longtime and heavy user of BeyondCompare 2.x in Windows. For me, BeyondCompare is the gold standard of file/folder comparison apps but it's Windows-only and I only use Windows for work -- and, even then, only when I must. Like when I need BeyondCompare.

MY NEEDS
Folder comparison is a must. File comparison is also a must. I'm particular about having a certain feature set and DeltaWalker fit the bill so I bought it via this promo today.

GOOD SUPPORT
I got a prompt response from Deltopia support today asking about Word doc comparison support (Araxis Merge has a filter that strips the text out of Word docs so they can be compared.) Deltopia support told me they've already started on Word and PDF comparison support and that they'll definitely have it out by the end of this year (2008). That broke the approximate tie (ignoring price momentarily) between DeltaWalker and Araxis Merge.

USER INTERFACE COMFORT
Although Araxis Merge uses native OS X controls and DeltaWalker is apparently a Java app or uses some other toolkit I'm not familiar with (the controls and icons didn't ring a bell), I was surprised that I found DeltaWalker instantly and consistently more comfortable than Araxis Merge.
So in the end it came down to more than just Araxis' huge price vs. the more reasonable DeltaWalker Personal price: I actually was more comfortable inside DeltaWalker.

THE OTHER TOOLS
The other tools I've evaluated didn't come close to the top two, DeltaWalker and Araxis Merge. And DeltaWalker got my money, without hesitation.