I triple boxed FFXI about 8 months ago, and it was a blast. Only reason I stopped was because of friends pulling me back to WoW.
The huge benefit about FFXI is you can actually multi-box melee classes effectively since they will run and stick onto an enemy.
I used all seperate PC's for each character but being able to make groups of the fly with yourself and just invite whoever makes the entire game soooooo much better! So many people cling onto you just for the fact you are your own self group which is a huge requirement in that game. I had a bunch of players on the game who would not level a certain job until they saw me online and wanted to join :P
Basically FFXI is awesome, and I still miss it.