I'm using the latest build, 10586.63 on my Lumia 830.
Regardless if my phone was on WP8.1 or in W10M, my phone could only last less than a full day (from 7.30am to 8.00pm), but that's alright because I am a heavy user (checks Instagram, Facebook, Twitter, AppRaisin, Windows Central, Store for updates, and repeat the cycle once more, and I do this about 6-7 times a day. I spend about 15 minutes doing all of these for each 'session'.).
But what I've noticed is that the battery life of my 830 is slightly better on W10M compared to when it was on WP8.1.
Usually, after doing all of the stuff I've listed above during my normal day, the battery on WP8.1 would have about <10% battery. Now on W10M, I'd get about 15-20% battery left.
My phone is on WiFi almost all of the time as my accommodation and university has free WiFi, but outside of those places I'd turn on my Cellular Data (I usually make it a habit to turn off the data whenever I'm near a WiFi area as I'd like to save on bandwith). Also, I set my brightness to Low all the time (unless I'm using my phone under direct sunlight), Location turned off, and all my apps are running in the background save for Readit.
So, yeah. I guess W10M has been kind to me? :3