I'm typing this post on a G3 iBook running OS X 10.3 and Camino 1.6.8.
I've not used this laptop for a while, it sits in the kitchen and Emily owns that space. So I was surprised to discover that it works very well as a web client (though the battery life is rather minimal -- the LiON battery died years ago).
The secret, of course, is to know when to stop upgrading. Theoretically it could run 10.4, but I know that would be slower than heck (it came with OS 9 and, I think, 10.1 or 10.0). Camino is a beautiful browser and this version runs very well on 10.3 (even has Expose!) . The iBooks always had great radio reception, and the 802.11b Time Capsule connection works (though there were some oddities initially).
Google, of course, is doing the heavy lifting. They keep getting faster, so my iBook keeps getting faster. That's the Chrome OS promise.
It's not just the iBook. Upstairs, with some hardware fixes and a few workarounds, my ancient XP box keeps getting faster with each release of Chrome. (My G5, however, is kinda slow with 10.5. I should have stuck with 10.4. The MacTel transition shortened the G5 lifespan.)
Computing ain't what it used to be. Feels to me like we need one powerful machine to manage photo, video, backup and file services, and everything else can just coast ... (Wii for games.)
PS. Considering the build quality of many netbooks, an old laptop with a newish battery is pretty price and performance competitive.