The FIOS app is excellent- except that it is usually redundant.
The FIOS app allows almost every cable channel- except the broadcast networks- in a quality as good as regular programing.
One could save the cost of renting a STB. What makes it redundant is that for the sets it comes with, you would attach a STB for DVR, VOD, etc.
I have a small TV in a home office tuned just to financial and news channels. If I knew what the app was capable of before I recently bought a TV for that space, and if it was available on a much smaller set, I would use the app and save the cost of a STB.
I can imagine an instance where I would use the app. Our DVR is capable of recording two channels but you can't watch anything else live while it is recording. If needed, we will now be able to watch something using the app.