I have a Nexus 5 which was pushed the Android Marshmallow 6.0 update yesterday.
If you are wondering what Marshmallow will do for you and whether you need it - I don't think most people will find it a very a spectacular improvement in most respects - but the big deal is that battery drain on standby has been dramatically improved.
Google call this new functionality 'Doze'.
Previously I lost around 2% per hour - maybe up to 20% of battery overnight which would be around 2 days standby from full charge on an unused phone.
First indications are that on Marshmallow the overnight drain is around 4% - which indicates a massive improvement in battery life on standby!
I would extrapolate up to over a week on standby for an unused Nexus 5.
These are the last two overnight performances captured in a Battery Monitor Widget App.
Before Marshmallow Doze:
I cant work out whether to say "Congratulations Google" or "Why didn't you do that sooner?" 🙂
Footnote: I have read that Doze uses the phone's accelerometer to go into a deeper standby mode when the device is not moving - so I expect that a phone on standby in a pocket or car may not see quite such good results...