Working with TomEE over the past six months or so has been fantastic in comparison to WL, WS, or even JBoss. My only complaint would be the documentation. If you know what to ask for, you can get answers very quickly on the forums, but so much of that should be available in some sort of document. Searching the TomEE/OpenEJB web site often results in out-of-date pre-TomEE information.
On a related note, I have a bunch of other developers that have been working with our old app servers and have some knowledge of J2EE 1.4 and are just now getting their feet wet with JavaEE 6 and TomEE. Does anyone know of any formal JavaEE 6 training out there that uses TomEE as the app server for the labs?