I had the same problem after upgrading to 6.0.1: battery life between 3(!!) and 5 hours a day. I went to the Genius Bar and apparently this is known firmware bug.
Their solution was to have me restore the device without using a backup. Basically wipe the phone and reinstall the OS, then set it up as a new device. That was a pain in the ass, believe me. Even with contacts and other settings synced to iCloud, I had to restore all my apps and then reorganize them, log back in to everything, restore my preferences, etc.
Battery life is now better, between 6 and 7 hours, but not nearly as good as it was when I first bought the phone (8-9 hours). Here's hoping for a permanent fix in 6.1.
Harry, if you rely heavily on Maps do not upgrade. Google is supposedly "putting the finishing touches" on their new mapping application, but until then the data is just plain unreliable and lack of transit directions is horrid.