In my former house, we switched to portable units for two rooms a while back. For the main room, we were able to buy a larger unit (as the windows were quite small). I can a 20 Amp circuit just for the portable unit for both rooms. The units seemed to work really well in both rooms and are pretty quiet and easier to install. They really not that "portable" though, as you still need to insulate the windows to prevent drafts. They're also more expensive for the same output. That is, the window units seem to be more powerful for a lower cost. Having said that, I would've gotten all portables after a while.
(Actually, we had everything electrically ready to install for central air; I would've done that, as I think the window/portable units are too expensive to run relative to central air, although the initial cost of central air is much higher.)
