Although I have been paid to work on both the Enhydra Enterprise Java application server and on the JBoss based Jaffa framework, I must admit that Tomcat has always been my favorite platform: lean, and add just what I need.
As a result of my 'build up just what I need' preferences, I just got around to experimenting with Glassfish. The Update Center is great, and as more instant install frameworks and applications become available, this will be a time saver. The web admin tool is refined - no complaints there.
I do have one complaint about the 200 MB resident memory footprint. A lot of what I do involves deploying to low cost servers, often semi-managed VPS systems. Smaller memory use is cheaper, but for most large server deployments, an extra 100 MB makes no difference.
I thought that it was very cool that one of the available instant install components is JRuby with the most excellent Goldspike. I have written before in this blog about the ease of running Rails web apps with Goldspike and JBoss. With Glassfish and the Update Center, it is even easier. Cool stuff.