My M9 is SIM-free, but the experience will probably be the same for VF-branded versions.
The update comes in two parts, the first being a preparation run. This is about 300kb, but took about an hour to install overall, a lot of which was spent optimising all the 200-odd apps after the final restart.
The next stage (1gb+) is the real deal and took about 2 hours. Only 85 apps were optimised at the end of it. For this reason, I'd recommend having the charger plugged in during both stages. Updating is an intensive operation and there's no way of pausing it or monitoring the battery state while it's running.
As with all updates, there are times when the device appears not to be doing anything. Be patient. Right at the end, after the final restart, I did have a nasty moment when the phone appeared to be stuck on the splash screen. Turning the display off and on produced the lockscreen and swiping that away produced ... the splash screen. After few minutes, however, it went into the homescreens and all was well.
One of the first messages was that I needed to update Google Play Services. Tapping on that simply gave me the "deactivate" option. Most things seemed to be working OK and messages were coming into the Gmail app. However, app updates and the Google Play store seemed sluggish. After 10 or 15 minutes, I did get Play Services to update and it was a lot slicker for that. I think the moral is probably to be patient and maybe just let the thing sit for a quarter of an hour or so before trying to do anything else.
The next notification was that HTC backup would no longer work. You can restore from a previous one, but there will be no new ones. Go into Settings|Backup and Reset, tap on Account and choose your Google one. Next, turn auto backup on - it's off by default. Whether Google back up or more or less than HTC I don't know but, if you change to another device later (:smileysurprised:), your M9 will be one of the options you can restore from. I've previously done a cross-brand restore and it works fine.
I have My Data Manager installed and it prompted me to alow it to access my usage (part of the Marshmallow permissions thing). Some other apps will ask for access permission/s the first time you use them under Marshmallow.
When I did the update on my M8, one app crashed on opening, but clearing its data in Settings sorted that. The same app was fine on the M9.
I haven't (so far) had to do a factory reset and I prepared the ground by clearing recent apps and restarting the phone before the update so as to give it as much of its resources as possible. Under Storage in the main settings menu, M has the option to clear all your app caches at once, so I'll be using that as well in future.
Someone commented that M handles external sd cards differently and that theirs was now part of internal storage. Mine is labelled differently and I have the option to format it as internal, but haven't gone down that route and it wasn't the default. I did once use a Vodafone-branded device that had little internal storage and made the sd card its own in something like this way. It was a neat trick that expanded the available pool, but I may not feel the need on a 32gb M9 (about 14gb is in use, mainly by the OS and apps).
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