I started with MasterJuggler in Classic (wasn't classic back then) and moved on to Linotype FontExplorer for OS X after deciding that I couldn't stand Suitcase - I used it for the longest time, but it was so inconvenient. I was relieved when I discovered FontAgent Pro. It acts like MasterJuggler -- invisible until you need to change a setting or add fonts to the library. It is also very stable and better at auto-activation than the other applications. I couldn't work without this program - there's no way I could go back to FontExplorer.