zip/gzip are
general-purpose compression formats that perform well on a wide variety of data. Occasionally, someone comes up with a better compression algorithm that wrings out a few extra percent, but most people don't bother switching.
Pack200 is different. It is optimized for
Java class files. For class files, it can achieve dramatically better compression than zip/gzip. According to
http://java.sun.com/j2se/1.5.0/docs/guide/deployment/deployment-guide/pack200.html#pack200_compression,
"One can expect compression to 1/9 the size of the JAR file, if it contains only class files and is in the order of several MB." I tried this out with a couple of examples, and indeed, the results were excellent.
However, if you use Pack200 for other formats, such as image files, the result would not be impressive.
What's not to like about Pack200? Since older JVMs don't know about it, you either have to make sure that all users of your applet/web start app have 5.0, or you have to rig the server to serve up either a traditional JAR or a Pack200-compressed one. See the above link for details.
Cheers,
Cay