The update process went smoothly. The download took about 1 1/2 hours, then Lollipop started installing. The S5 rebooted and when the AT&T logo screen came up it sat there just long enough to make me start thinking it was a freeze, but then a message popped up to say that Android apps were updating, and it provided a running count as the apps updated. When that was finished, Goggle play said it needed to update and did so. That was it.
My S5 was plugged in while downloading the update. By the time everything was finished, I still had about 90% remaining; however, when I checked the time left, only 3+ hours remained. I wiped the cache partition and then I spent quite some time checking out various apps to see how they were working, then I rechecked the time remaining on the battery and it showed 16+ hours. So far, after 5 hours, everything still seems to be working smoothly, no bumps in the road.
Lollipop looks pretty good. I didn't think I'd like the white background but I find that I do like it. Lollipop seems to respond faster too, and all my apps seem to be working just fine.