I tried Suitcase Fusion. It didn't even have a mechanism of simply showing the fonts in WYSIWYG way! And dragging fonts to "sets" was a pain.

FontXPlorer from Linotype is nice, but it leaves a lot to be desired, for instance when I activate a font, I want it enabled FOREVER, not only for my current session until reboot, thankyou. Scoured the preferences but some basic features are missing.

OSX's default "FontBook" is nice enough for simple people but not for typophiles.

Enter FontAgent Pro. Everything you want. Excellent interface, shows fonts easily, groupings/sets work like a charm, and the preferences are clearly laid out so you KNOW what's up.

Highly highly recommended.