I had Comcast years ago and have FIOS now. So I can't tell you how good or bad Comcast is now but on FIOS, their internet service is very stable and fast, HD tv is clear with no pixelation as it's fiber and they use no compression, land line phone service is excellent with all services included, call waiting, forwarding, tv on screen display, etc.
They have issues when a billing problem occurs, so you have to hope they get it right. They took some channels away from my tier of HD recently even tho I am on a contract. I don't like that. And they keep changing the tv packages and raising the prices, I don't like that either, but I think all the cable companies are doing that now.
So, FIOS quality = Excellent. FIOS Customer Service = Poor.