A while ago I came across this post by John Resig pointing out that although packing reduces the file size, it takes longer to ultimately execute, due to the repeated unpacking process every time the script is used.
After reading this I started using the Yahoo YUI Compressor to minify all of my plug-ins, and it worked well, typically reducing file sizes by 70% or more.
Looking through the jQuery 1.4 release notes this week I noticed that the jQuery team have switched to Google Closure Compiler. This really is a great tool, for example it reduced my Exclusive Checkbox plug-in to only
191 211 bytes and the MapReduce plug-in to under 1k in size! You need to be careful when using the compiler with the advanced option (due to the possible renaming of public methods and variables), but this doesn't apply to plug-ins because of the use of a closure – perhaps this is the reason for the name of the compiler.
Update I had to abandon the use of the Advanced option for the Exclusive Checkbox plug-in, increasing the size from 191 to 211 bytes ...