I do have FiOS and it is battery backed up, but you are responsible for replacing and paying for the batteries. They don't even do a good check as to when the battery won't take a charge. Even with battery, you get significantly less than a day's worth of standby time. POTS gave you a minimum of 24-hours.
FiOS is moving their customers to VoIP-based telephony, and it's generally not very good. Lots of times where calls don't work, no dial tone late at night, etc. The old POTS circuit (which they removed) was archaic, but had much better up time. On the other hand, it's cheaper and they throw in all the "features" you used to have to pay for like call waiting, caller ID, three-way calling, etc. Not very hard to do with VoIP, of course, but at least it's cheaper. I looked into dropping the voice part, but it came down to less than $10 a month for unlimited long distance, voice mail, etc. Hard to say no to that, since it still sounds a ton better than AT&T cellular.