I still use a 12" 1.5 GHz G4 with 1.25 Gb RAM running Tiger as my everyday machine, even though I have a 2.33 GHz 15" MacBook Pro and other, newer machines hanging around.
The 12" form factor is great, particularly if you travel a lot, and the performance is fine for most tasks. At the moment, I have 14 Firefox tabs open, iTunes playing some music, the BBEdit text editor running, and a Photoshop Elements 3 window open, and it is running just fine. I also use Office 2004, Safari, Opera, and a few other apps routinely.
It's a tough little machine, having survived a few hard falls with no consequences besides a few major dents.
The only serious negative about the machine (aside from the fact that it is a little more than four years old) is that it does tend to run hot, and, as a result, the fan runs often -- and loudly. Running things like Flash widgets or Adobe AIR apps (e.g. TweetDeck) seem to kick up the fan activity quite a bit. I am noticing that Flash-intensive web pages are less snappy than I'd like, but they're not too bad yet.
One final point - given the age of the machine, you may need to buy a new battery (appx $120) if you plan on running it untethered. If you can, ask the seller for the number of battery charge cycles and/or the charge capacity. (You can get the info from the "Power" section using the System Profiler program or by running a utility like
coconutBattery
). Once a battery gets much beyond 150 cycles, it can lose its ability to hold much of a charge. A new battery for this model will have around 4400 mAh. Anything above 3000 mAh or so will still be fairly useful. Much less than that, and the battery will last less than an hour.