Odyssey puts out a really good technical manual on the PC680. Most of the questions and issues in this thread are addressed. In particular, the issues around charging between 14.2 and 14.7 are explained. There is a really interesting chart showing the impact of charging at the low end versus 14.7 which is the recommended charge voltage.
I have used three PC680s in my RV7 over the past 8 years. Two were changed "just because" ... one was dead in the saddle. Have seen a lot of them at KORK (we have 22 flying RVs and 21 are using PC680's) and generally, we have seen good service life out of all the batteries.