I have been using it for years, it works very well
What Piotr Konieczny wrote about this project moving June 2009 appears to be untrue. It seems the project he points to used to be called "PHP Thumbnailer" which then change it's name to "PHP Thumb" (note the space) and even uses "phpthumb" (without the upper-case T). phpThumb (this project WITH an upper-case T) on the other hand has a completely different bunch of source files, is associated with different people and and was last updated August 2011. Unless the author of this project mentions differently, I believe these are two separate projects with similar names.
There are no 4 star reviews.