http://www.wekeroad.com/blogs/PermaLink,guid,3cbbc786-6ccc-4e35-917b-33148ef7655d.aspx

Definitely worth upgrading (and not just because I spent a lot of time working on this release). :)

In addition providing minimal benefit, it will, in certain cases cause problems in the output pages. I tried it a couple months back and ended up ditching it after a few minutes. If I recall correctly, it created some problems with inline javascript, which doesn't like having certain whitespace removed. As the previous poster alluded to, the real bang for your buck is in gzip compression, which can easily be added to your entire site, or selective pages:

http://www.codeproject.com/aspnet/HttpCompressionQnD.asp
http://www.blowery.org/code/HttpCompressionModule.html
http://www.microsoft.com/belux/msdn/nl/community/columns/desmet/compression.mspx

AJAX and JavaScript should be made into one category. That's what the J in AJAX stands for after all.

Sergio also has a really nice guide to Prototype:

http://www.sergiopereira.com/articles/prototype.js.html

Also, while not explicitly mentioned in this post, disabling Word Wrap has improved performance for many users as well.