2x machines here with IS Boxer. It works amazingly well.
3x accounts on my main pc
2x accounts on my older pc.

I use two simply because my main machine can't handle all 5. Contemplating upgrading the cpu in the second machine to to quad core so I can put 3 or 4 on it.

You can use a little utility called Synergy that lets your mouse and keyboard from your main machine control the second machine, you just scroll your mouse off the screen into the next desktop and your controlling it.

I only do it when messing with talents or equipping gear generally. Synergy starts up automatically so I don't ever touch the keyboard/mouse on the second machine anymore. I think there is a better program than Synergy for doing it, but can't remember the name and never got around to trying it.