The home button on my wife's iPhone 4 wasn't working. It was behaving as if it was stuck down. You could press on it, but it wouldn't move like a button normally does. I had found other references to software related issues on home buttons but not hardware ones aside from a few rare instances.
It went well. No problems. The guides do not properly prepare one for the true tiny-ness of the screws inside.
I believe that the problem with the home button in this case was due to dropping. In the picture in step 33 of the iPhone 4 Home Button guide (http://www.ifixit.com/Guide/Image/meta/X...), there's a metal circle behind where the home button sits. In this phone, that circle was no longer centered in the space provided. It had shifted over to the side quite a bit, and I believe that interfered with the operation of the button. My only guess as to how this happened is dropping, as it's only stuck in place with some adhesive, so a good drop could potentially knock it out of place.
I decided to replace the button in addition to re-centering that circle as I didn't want to have to put it all back together and potentially take it back apart if I was wrong.