My power button wouldn't work. I would press it, never feel a click and never could lock or turn off my phone.
It was time consuming, yet mostly successful. The 2 screws that hold the power button against the case were the most difficult for me to screw back in. I actually lost one in the process... but I put it together with the screw closest to the outer edge out. Hopefully it can survive without it or at least until I can find a replacement screw.
I took my time. It's a stressful feeling for me not to have my primary source of communication for a temporary time, even if just for 3 hours. But my mentality was taking my time and doing it right (or mostly right), otherwise I would have been without it for much longer than 4 hours if I damaged something more serious than losing a screw.
ALSO, I was using a guide for a non descriptive "iPhone 4." I have a Verizon iPhone 4... so in the last few steps of the guide I was using, I was getting frustrated that I couldn't follow along... turns out there is a corresponding guide that was specifically written for the CDMA version. So if things aren't going smoothly, look for an alternate guide.