The past 6 months or so we have been working on a 126.96.36.199 upgrade. Last weekend we finally upgraded production over a period of 48 hrs. (That includes rebuilding test environments for testing.) The upgrade went pretty smooth and there haven't been any major issues on the techstack side. We have had the usual jinitiator issues and I believe there have been some minor application issues, but otherwise i'd say a very successful upgrade.
Going into the upgrade we expected it to about the same level of effort as our 11.5.8 to 11.5.9. However, that wasn't the case to say the least.
So hopefully i'll have more time to post in the new year...!