The T40 Thinkpad is the best value in a thin and light notebook computer available. It's fast enough to hand every task I ask it do, and light enough to carry around in a backpack all day when needs be, or travel in a saddlebag on the Harley. I bought one to use for my yoga teachers website and personal use and liked it so much that I bought another for my day job.They both came from eBay sellers and there have been no problems with either machine. I'm writing this  guide on one now.
    This is a business computer, not a consumer product. It was meant to travel through airports, be carried around, and be running all day without overheating and quitting. The windows XP Professional operating system is both stable and powerful. Some have built in 80211 G wi-fi and some need a nic, ask if it is not in the description. A few come with built in Bluetooth and IR Most of these computers were built to specs for universities and corporations so they don't all have identical options.
     The trade offs are a relatively small screen, by todays standards, and the battery life isn't as long as the new dual core processors. That's alright for a computer in the  $450.00 range. The mainstream computers in that price range are very disappointing if you plan to do much serious work with them.
     Parts for these computers are inexpensive and plentiful because IBM sold or leased lots of models that with pieces that interchange. Batteries are very inexpensive due to the shared model usage and number of computers in the market.
     The bottom line is that if you don't need to have the very latest notebook, new operating system, or a flashy case, this computer is a good choice. It is a solid ,durable worker that is ready to go anyplace. As with anything you buy on eBay, ask questions and know your seller.
     I realize that this guide is an opinion but I hope it helps you find a good value on a good computer.