I would also like an update on the potential battery life difference between the various DE.
Unity is definitely the most power-hungry of the lot and KDE 4 has matured very well now, but hungrier DEs may also be more power-efficient.

I guess everyone would have to try on his/her personal config what gives a better battery life by streaming one playlist from Youtube for instance. General benchmarks from Michael won't be of much use since consumption does depend a lot on the hardware (HDD/SSD, quantity of RAM, deactivating USB 3...). Phoronix is not really the place to tweak and test all these.

Plus it is now pretty common to find middle-range laptops with 5h+ of battery life so anything below a 10% difference won't really matter and is more about setting up a battery-saving mode than choosing a DE. It is really more about the quality of the battery over time - and how to optimize it (keeping it in the 30-80% as much as possible?).