I flashed the update today and I can't say much about how much battery life has improved because when I unplugged the charger it was at 70%.
Project Volta isn't some silver bullet regarding battery life but if developers start taking advantage of it we can see dramatic improvements in battery life not just on the Nexus 5, but many other devices as well.
The new power saver mode is pretty awesome btw. I'll report more detailed info on battery tomorrow.
Pretty much this. Lollipop will be providing some improvements to battery, but don't be blown away by dramatic increases to usage time between charges.
Also, for users comparing the Nexus 5 battery life to other devices, remember that it's wrong to compare devices that have significant battery capacity differences. For example, the 3000mAh size of the One Plus One will provide longer battery life than the Nexus 5's 2300mAh, simply due to capacity. Yes, software, device settings, etc. have a lot to do with it as well.
I'll be installing the update soon, and will report my experience.