I use a barebone system as well as a tower. Oringinally I bought the barebones sytem for work related tasks. All I needed to run was MS Office, Outlook, Internet, dev tools. It catered for this task nicely.
Purchased it in Novemeber. Over Xmas I belatedly bought Warcraft III as I wanted to get back into that game (one of my all time faves) and the barebone was able to cope with this no problem.
I netwroked it to an existing tower for Warcraft against a friends, and still handles that just fine.
The system I picked up had room for an AGP card, so I've put one in and game performance increased accordingly.
As for portability, this thing rocks. I've carted it to parties to use as a jukebox as I've backed up a lot of my CD's onto it, and with Winamp on random play, nobody is fishing thru CD collections all night, and it sounds fine attached to an Amp (obviously the MP3 format lowers the sound quality a bit, but it's a party - who cares!).
I initially thought it would be a waste of money, but really, it's one of the best things I've bought. Handles a DSL connection just fine! Hardcore gaming might run it a little hot, but I'm a strategy/RPG fan so not pushing it to it's extremities.
OK, it will never be a high end machine, I admit that, but I do have a tower which is under constant upgrade, and the barebone was a cheap compact machine that serves a few alternate purposes to just sitting under a desk.
As long as there is a monitor at your destinations, it's a cheaper option than a laptop.
Just my 2 cents worth!