I'm juggling characters on different servers, and it's actually not bad to keep all the balls in the air.
Verdha on Lamia is my original character, in the FC with all my FFXI friends from the NoSephiroth Dynamis LS (Catwho, Illsaide, and so on), and is focussed on progression and fun stuff with the game itself.
Waenlona on Balmung is my RP character, in a heavy RP FC with Hyrist's multiple characters, and focussed on RP first and everything else second. I still find she progresses pretty fast.
Kekerana is a lalafell (because lalafell, right?) I've just created on Sargatanas to drop in one or two times a month to visit with the friends from my original Seraph FFXI social LS.
My week consists of "Verdha days" and "Waenlona days". Waenlona Days are chosen to fit around the FC's main RP schedule, while Verdha gets the other ones. A regular routine helps plan out activities for each of them and keeps them both in play without neglecting either. I only pop on Kekerana very occasionally, but it makes a great break.
It's much more interesting with alts for me, partly to experience the game in different ways (RP vs Progression), and different storylines (three characters, three starting cities), but also because I find I get into the characters in a very different way. Verdha is a career MNK with a WHM sideline, never could get the hang of SCH. Waenlona is a full time SCH, pretty darned good at it, but struggles on WHM and has no interest in DPS or Tanking. Verdha and Waenlona both are poor BLMs. Kekerana is a classic FFXI tarutaru BLM, a creepy looking guy with an attitude that loves to blow things up and figure out which side they're on later. And none of those differences in how I play the classes were a conscious choice -- I just find that on Waenlona, I play SCH better than anything else; on Verdha I can't get into the swing of SCH but WHM is fine and MNK is career, and only on Kekerana can I nuke effectively.
One catch is that Balmung is not open to new character creation, so I had to create on another world and transfer over. But it was well worth it.