I love the persona system.
Other options for "playing something other than my main" have been poor fits for me. I have a second account but I loathe boxing; I've let that character languish in the 110s and the account lapse to silver. I'm aware that boxing is the path to power but it isn't fun for me. And traditional alts just feel like wasted time. The biggest advantage of alts is discrete lockouts, but I've never taken advantage of that. My highest level traditional alt is also largely abandoned in the 110s.
My main is a 125 shaman in very close to max current raid gear. And I love the persona system. I have a 125 rogue. My paladin is about to start the tough run (111 to 125). My ranger is 105. In total, I have 9/16 classes unlocked through this system, including my main.
The persona system lets me explore different classes and play styles while maintaining the advantages of having dedicated years of play to a single character. I can farm old raids for leveling gear for my personae. They benefit from my huge back catalog of clicky items. When they hit 125, they even get to wear (most of) my raid-earned nonvisibles.
And to some extent, the personae benefit my main. Having the rogue and its lock picking finally let me get my shaman the IVU earring from VT. The ranger in particular is helping me work though my shameful backlog of Hunter achievements. My mage can make pet toys for Dogdog. The rogue has given me direct access to the last two tradeskills (poison, tinkering). My enchanter will eventually, um, enchant stuff for me. Most of the time, I still play my shaman, and I raid on it exclusively. But when my friends want DPS, I've got a rogue; when they need tanking, I'll have one soon. When they want to hear me cackle maniacally while I aggressively CotH them around, I've got a mage!
Plus, for giggles, I'm running all the epics. All 9 of my current classes have their 1.0s. Eight of them have 2.0s done; the paladin has only a 1.5 because I haven't dragged friends to Felwithe for the "need six people right here" request step. Someday, I'll have all 32.
Is the system perfect? No. No, it is not. The 111-125 leveling stretch is unnecessarily punitive. And inventory management with one persona isn't as bad as some people claim, but with a whole stable of personae, it's pretty grim. But those feel like the sorts of things that can be improved upon in future. I'm not entirely unsympathetic to the idea that it weakens the RPG aspect of the game in terms of character identity; I don't want to diminish the experience of players for whom that's really important (and I indulge in my share of pro-iksar propogandizing!) but for the most part, I feel that ship has sadly sailed long ago (maybe things are different on FV?).