After reading about a lot of people having trouble with this update, I'm come up with a plan to upgrade from Kit-Kat (4.4.4) to Lollipop (5.1):
1) Factory reset to clean 4.4.4: This will delete all of the non-Google (except for Sprint, grr) apps, thereby hopefully avoiding the "Optimizing Apps" problem people had
2) Update to 5.1
3) Factory reset (just in case)
4) Clear cache partition
5) Remove default guest account (I seem to remember some people had trouble with this)
I'll let you know how it goes! I just bought the phone in January (found a new one in box for $200!); as my old one's screen broke, and there's no way I want to buy a phablet (Moto X Pure); so it's still under warranty, hopefully it doesn't come to that.
werent they sold to someone then sold to someone else recently?