Inspired partially by Doctorow and Pilgrim's switch to Linux from OSX but mostly out of a desire to resuscitate ancient laptops for use in situations where they may need be disposable, I installed Ubuntu (Dapper Drake) on a Thinkpad A21e (600MHz Celeron, 256MB RAM, 10GB HDD). It was dog slow and I couldn't find a theme that didn't seem to waste lots of my limited 1,024 by 768 screen real estate. (I'm the kind of person who changes XP's theme back to 'classic' because the task and title bars are, like, 3 pixels thinner.)
So after spending the better part of a day install and compiling new applications and drivers, I blew away my Ubuntu install and tried Xubuntu, the "lighter, more efficient" version of Ubuntu that uses the snappy Xfce desktop instead of GNOME. Xubuntu was what I was hoping Ubuntu would be, providing a capable, useable environment in which to browse the web (with Opera), run a few applications like GAIM and Thunderbird, and still bop around in the menus without waiting for the icons to load.
If you're looking to make a man out of an old computer, try Xubuntu. It's a trivially easy install. That said, it's still Linux, which no matter how hard it tries continues to show up to the usability prom with its dress tucked into its hose. The first time apt-get failed I knew I'd be administering my desktop again instead of just using it. Are the Linux developers too oblivious to the real world to realize that there is never any reason a user should be on the command line anyway? Like, never ever?