my XS has been having a problem the last couple of weeks, gets down to around 30% then powers off. Plug it in, comes up at 11% then charges to 100% in about half an hour. Then stays at 100% for half a day or so, even with reasonable use. Annoying, but not unusable. Seems to be something to do with the power management software. What is more annoying is that the the 30% gradually became 40%, which is now around 55%. Still get a good day+ of use out of it, just means the battery icon is pointless.
What is really annoying is that having sought out "expert advice" i did a full factory reset and set up 'as new' so not from backup. Which means i have lost a fair amount of stuff that (by my own admission I forgot about backing up (some notes, saved games, that sort of thing)). Thought it looked like that had saved the problem, worked fine for a few days, but this afternoon, bam, shut down around 50%, charged from 11-75% in 20 mins......
so not only do I still have the issue, but i've lost a load of my junk into the bargain. For the record this is the first (even minor) issue that I have had since my iPhone 3G (not that i get a new one every year).