No benefits so far, only cons, at least on Nexus 5.
1. The new UI sucks ass
My wife has it on her nexus 4, and I have to agree with you here. The look is just terrible. And WTF is with the new icons on the navigation bar? OK, the old icons might not have been 100% intuitive, but an arrow that turns around and goes back is at least someone obvious to most people. A button shaped like a house is at least fairly obvious. Both of those, once you click it a few times and see what it does, anyone should be able to associate the picture with the action. The overlapping rectangles would be less intuitive to anyone who has never used a desktop OS before, but at least it makes sense to some people. But the new icon...WTF do circle, square, and triangle mean? The only one with the slightest bit of intuitiveness is triangle...it's actually an arrow pointing left, which means "back", but the way it's drawn makes it extremely difficult to associate it with an arrow.
3. The gallery application is also now ruined, as it forces the user to accept G+ account, or he can not access his pictures anymore. Way to go Google, taking pictures as hostages really makes the G+ positively accepted.
Didn't realize that, since we already use G+, but I was sort of shocked when I couldn't find the gallery app the other day and had to use Photos
4. When battery gets below 15%, the system starts nagging via notifications, making noises, and bugging the user, even if the device still has hours and hours of standby time left. Why not complain once and let the user connect charger when he prefers?
Not sure what all you mean, but when I had my nexus 4 with kit kat, it did something similar. It would give you a popup with noise and vibration when you hit 15% and then another 4 or 5 alerts at various percentages. Though I don't recall if it did that when the screen was off, so maybe that's the difference you are noticing.