They are cheap, they are sturdy, and they are fast.
The design is one that works with the user. Even though it doesn't look 'good', I love it and don't think I'll ever be able to change.
This video embodies my opinions of how great the old thinkpad designs are: https://www.youtube.com/watch?v=doEZMNXz1JY
Honestly, they're pretty comparable. The Dell is an attractive package, and has a better display, but the ThinkPad wins the input battle (IMO) with it's keyboard (curved keycaps, better travel) and trackpoint. I also have more ports and RAM available (one stick is soldered, the other is user-replaceable).
Another thing to take into account is that you can get the Dell w/Linux out of the box. In my case, I had to pay the Windows tax and then install Linux.