Yea I would love to see any kind of data, graphs, explanations, war stories, etc on this. We've invested a lot of manhours in tuning our CMS GC settings, so I need more than "its the latest" to justify a few weeks of testing & rollout.
Anecdotally, I've found the CMS is still a bit better (10-15% faster) than the newer G1, but the G1 has some features which make me inclined to use it. The G1 will return memory to the system, which I have never observed the CMS do, despite having tons of unused memory in my Java applications.
I also found the CMS collector to be both faster and generating latency numbers with less variance for our application. But, I last checked a good year ago and things might have changed since then.
http://docs.oracle.com/javase/7/docs/technotes/guides/vm/G1.... - at the end it says "G1 is planned as the long term replacement for the Concurrent Mark-Sweep Collector (CMS). Comparing G1 with CMS, there are differences that make G1 a better solution." - as G1 shipped with Java 7 I think it's stable, otherwise they probably would have waited until Java 8 imho.
"Stable" != "high performing", especially under all possible allocation & garbage generation patterns.