I bought the T400 after extensively using a Dell for the better part of high school an it's definitely been a pleasure working with it. I'm currently a junior at MIT and the T400 has handled everything I've thrown at it with ease. With 3GB of RAM, it handles all of the major engineering and math applications without any problems. And it's great because I don't feel like I'm going to hurt it when I throw it in my bag along with other books and gadgets.
Also, being an avid user of Linux, hardware can sometimes be an issue on laptops. Although I've had issues in the past, I didn't have any trouble installing Linux and it's many features on the T400. This basically just means the hardware is up to date and popular enough to avoid compatibility issues.
Other features that I like are the highly recognizable red track-point in the center of the keyboard. It makes working without a mouse a lot easier and faster, specifically it keeps both of your hands on the keyboard so you can type and navigate without moving your hands. The looks won't embarrass you when you pull it out in a big meeting, either. Nobody wants to stare at a glowing apple sitting across from you. :) Apart from that, the keyboard is great and I normally get around 4 hours with the 6 cell battery.
The only disadvantage so far has been that it is a little pricier than other laptops. On the other hand, I don't think I'll ever get anything other than a ThinkPad.
How helpful would this review be to a person making a buying decision? Rating guidelines
Sorry but much more information is required to achieve higher ratings. If you add anything to this review in terms of additional detail and opinion, please do not hesitate to drop a quick message in my guestbook and I will come back and re-rate :)