For me, that it. I've tested all of the ones I've found, I had these criteria:
* FAST (almost every photo organizer fell through here)
* Handle thousands of pictures
* DON'T mess with my folders (or «import» pictures to some stupid location)
* Tags are saved INSIDE the files (so that I can delete the database, or move the files to web storage)
digiKam does all of this. I was a bit reluctant to install it to grab the whole KDE (including Dolphin and KMail, WTF Ubuntu!?) as well. But as I said, this program is the only one that's actually usable with a huge amount of picture files. I don't know what all the GTK-guys are doing wrong - but F-spot will never be cater to the pros.
digiKam could have a better interface for tagging hundreds of pictures, I can't find a totally keyboarddriven way to tag pictures. But still, I'm extremely happy I tried it out although it doesn't fit in on my desktop looking all strange and QT-ish. ;-)