Jeremy Horwitz, in his iPhone 5 review for iLounge:
If you’re using your iPhone 5 in places with a very strong (4- to 5-bar) LTE or 3G signal, your cellular battery life may approach that number, but if not, the cellular antenna will struggle to maintain a signal, and fall well short. Because LTE and 3G/4G towers are in a state of build-out flux right now, our tests suggest that many LTE users won’t come close to Apple’s promised numbers.
I have no hard evidence to back this up, but my experience over the last few days seems to suggest this is true. When I have good reception, the battery life is as good or better than a 4S, with the added benefit of it being LTE. When I have poor reception, which I typically have at home and at work, the battery life is worse than my 4S. That said, there’s almost always a software update soon after release with tweaks that improve battery life.