Why?

OK, so you might be wondering: why am I doing this?

Well, I think the main reason is that I want to do something else in WoW, and I can't raid right now. If I knew I'd be able to raid consistently I'm sure I wouldn't be doing this.

But it's also quite fun to do instances without having to PuG.
As convenient as the group-finder is, I find I always seem to end up with people who come into dungeons over-geared and in a hurry. Teams are typically so overpowered that they flatten everything without needing to know how to play their class properly, or the correct tactics for each boss. And they're always in such a hurry for either exp or badges that they force you to rush through, rather than actually let me take them at a slower place and actually enjoy the game.

There are other fringe benefits as well. Five toons means 5 times the quest rewards, so I've had a lot of gold coming in recently.
Alas they eat through money as well though (getting them all epic flying mounts was quite dear).

Could I recommend multi-boxing to someone else?
Well, it's a very fun way to level alts. And now's probably a good time to level alts, with Cataclysm coming up soon.
But you have to be the sort of person who's good at thinking outside of the box and coming up with alternate ways of doing things. I've tried to cover the basics with this huge guide, but only really the basics. There's lots of speific details that are just too numerous to mention and you need to figure out for yourself.
If you do want to give it a go, use the recruit a friend scheme to get a free account and just try it with two at a time to begin with.