The documentation, although existent, is quite disorganized. By this I mean it is difficult and not always intuitive where the documentation for a specific and known feature can be found. There is not enough examples of how certain features work (like map() for example.) There are no tutorials that I can find and no mailing list. The homepage links in the PODs are broken. This lack of coherent documentation is my primary beef. Were it not for this my rating would be at least 4/5.
Other than this, the module works well and is even faster than GD in many instances. GD however is much easier to learn, again due to the superior documentation.