I did this last night. There are three small updates that come through first, then the main event, Marshmallow. I followed my usual process of closing recent apps to give the device as much of its resources as possible, but didn't restart as I hadn't done anything else at that stage.
The whole process, including optimizing over 200 apps, took about 2 hours and I had the phone plugged in all the time - it's a power-intensive job and there's no way of monitoring battery level, so better safe than sorry!
After the final reboot, there were updates to some of the inbuilt apps, mostly HTC, but also Facebook. I also got a warning about updating Google Play. Tapping this, however, only gave me the "deactivate" option and everything seemed to be working OK. It persisted so I cleared its data in the main settings and the message went away, It remains to be seen whether it comes back when I turn the phone on again later. This was the only odditity.
Apart from the obvious things like app permissions, the system doesn't look so very different. My icons seem to be the same, though I have noticed that some of the system messages come in slightly heavier type. Google Now has been replaced by Google Now On Tap and isn't accessed by an upwards swipe from the home key but a long press on it. This takes a little bit of getting used to, but I'm already liking not having it pop up when I scroll down on a long news story and start the movement too low down! When you first access it, you're asked whether you want to opt into On Tap and I did. I haven't used it yet.
After updating, I went through all my apps to make sure they worked. A few asked me to authorise their permissions and I also noticed that I needed to turn sync on again in Google Photos. Only one app (Radio Times Discover TV) crashed and I fixed that by clearing its data and starting again. I don't remember an obvious difference in the keyboard, but I'll look more closely.
All this gave the phone quite a workout and my initial feeling was that there was more battery left at the end of it than I'd have expected - about 70% rather than 50%. This isn't an empiric test and time will show whether the new OS is that much more efficient - though it's supposed to be at least a bit. With that much use, Doze didn't really have a chance to show what it can do. I'm also trying to decide whether I have slightly less free space than I did. I might have had 200-300mb more previously (?)
It's interesting that MM has come to the M8 before the M9 (the 8 was on 5.0.1, the 9 has 5.1.1). HTC say that it'll be on the M9 shortly. I'd have done it first on the older device, which isn't my main phone, in any case.
