We plan to begin our testing next week.
That said, the last "closest to stable" release of SEP 12.1 was RU2. Every release since then, I'm sorry to say has cause MORE issues than it's solved, RU5 especially., but even the many RU4 variants were a problem.
We're very hopeful that RU6 will be the first release since RU2 to warrant a mass upgrade on our part, by which we don't mean SEPMs and GUPs (that goes without saying), we mean have all our customers update all their SEP Agents. To date, we upgrade the back-ends, but are less compelled to update Agents unless good reason is able to be justified...far too many bugs the last few years.
Wish everyone luck with their testing, and if you hit something major, please share here.