I've seen a lot of implementations of thumb nail functionality in PHP, I've even written a couple of my own. I like this class thought. In looking over the code the first thing you notice is that it's clean code. I don't mean they use the latest whiz-bang method to cshave a few keystrokes.
This code is simple, nicely formatted and easy to read through. My only real problem with the code itself is that it is not terribly well documented. That can be forgiven since easy to read.
Cal talks about some of the other features of the class, including that it works with gif/jpeg/png images, makes it easy to set the quality of the jpgs, and makes it easy to display the dynamic images directly to the browser. Check out the demo for more.