I've had my Nexus 6 with Marshmallow for several months now and it's NOT quite what it's supposed to be. It DID start life doing what Google promised but as Marshmallow updated several times, that changed.
I WISH I could get Marshmallow OFF of my device (it CAME with Lollipop but updated about 3 days later). It ran FLAWLESSLY on Lollipop. Marshmallow is, IMO, one big giant P.I.T.A. and again, IMO, was released too soon. There are a LOT of bugs in it. Closing background apps is almost impossible without going through many gyrations. The memory that this release eats is ENORMOUS.
Most apps don't work with Marshmallow or if they do, very well.
I wish I could get Marshmallow OFF of my device and put Lollipop back onto it.
Again, it DID work as Google said it would when Marshmallow was FIRST updated over Lollipop but thereafter, no. Battery life DRASTICALLY shortened, permissions NOT disallowed until activated (it worked as they promised at FIRST, with updates, not anymore). My list goes on but won't bore you with it.
I guess my advice would be to hang on to Lollipop long as you can. There is ONE great thing that works with 6.0 and later and that is the NEW Google camera app. (which you must upgrade via Play Store). Other than that... I'm actually in here to seek advice on where to go for optimizer apps and security apps because I can't find any that work with Marshmallow (not fully, anyway... no browser protection on ANY I could find).
Saw this post and thought I'd just add my findings about Marshmallow but now must seek what I've come to find! How to make Marshmallow work properly (with security and optimizing apps of which I cannot find for 6.0).