Well it will be interesting when x265 (which is the only HEVC encoder I'm interested in as it seems poised to become the x264 equivalent) has had more time to improve, in the last tests I did (which were more than a month ago) it was worse than x264 (and consequently VP9) due to losing a lot of detail, but as stated the encoder is very young and also under heavy development, chances are I would already get very different results if I redid the tests today (currently keeping an eye out for some major improvements before I start testing again).
I was comparing 8-bit x265 against 10-bit x264 a couple weeks back, and was quite impressed - x265 managed to preserve quite a bit more detail than x264 in many places (the previous time I had tested x265 it didn't manage to do even that). However, there were two pretty glaring issues. For one, x265 sucks at dealing with gradients, resulting in horrible banding. Two, as pointed out in the OP post, x265 is slow - I was pitting preset veryslow against preset veryslow (used CRF with x265, bitrate-matched with 2-pass x264), and 8-bit x265 was 5-6 times slower compared to 10-bit x264. Working 10-bit x265 could potentially fix the banding issue, but the speed issue would likely be even worse. There's still quite a few ways to go before using these new formats will be actually practical.
