Coincidentally yesterday I decided to flash Marshmallow on my Nexus 7 (2013) from the Google images.
Since the device already had an unlocked bootloader, it wasn't too hard. In fact the hardest part was getting ADB drivers for Windows 10 since while the N7 showed up as a device for media, ADB could not see it. In the end I had to find a different set of ADB drivers for Windows 10.
Then this morning I received the popup notification that Marshmallow was available on my N5 (bought in Australia) and since it had gone okay on the N7 I agreed and 45 minutes later it's up and running. This is my second (the first being the N7) experience of this release so I can't comment much but on the surface it looks pretty similar to Lollipop. But what I am hoping for is the apparent increase in battery life through smarter power management etc, things that plague me on the N5.