After 18 months of use, my iPhone battery was finding it hard to last the day - I guess too many apps and a remote location.
Following your guide, the opening proved easy but the battery was glued in really strongly along a line underneath and towards the middle. I broke the plastic opening tool - does iFixit offer a metal one? Eventually it was levered out using a blade from the Driver Kit - easing from the outside side.
... Following that happy ending, I've now also replaced the battery in my wife's Nokia N8 for the same reasons and with the same ease thanks to your guides. Much appreciated iFixit!
For the iPhone, suggest try loosening the battery first from the (long) side which is nearest the edge of the phone - in my case the glue was under the other side so lifting this first proved easier and gave leverage to breaking the glue bond.