I'll be the seemingly lone dissenting voice from this all, but my wife and I did 2 seats (with our lap infant) as she was growing up and had good experiences where - as others mentioned - was all about 'survival'.
On the lie-flats, we did:
IAD-BRU
BRU-IAD
FRA-IAD
EWR-NRT
HKG-EWR
with our daughter ranging from ~9 months to ~19 months
And in addition we did 2 F seats on domestic planes (ORD-ANC and SJU-ORD for instance) on many routes from when our daughter was 3 months to 23 months...
All was good and turned out okay and I'd rather do that and save money/miles than buy three seats - but then again, i'm a cheapskate and got all those upgrades with instruments and never paid for F nor used miles.
Service on UA is and has been awful, so expect the worst and you'll be satisfied. Not many offers for staggered meals, crummy food, lack of empathy from the crew and a general apathy toward safety and the parents make it about the same in Y and F. The only difference for us was the seats and especially the amount of people sitting next to us, so if you opted for 3 Y seats, on most planes, you'd be in the same boat - a full set of seats to yourselves.
My recommendation is that if you are choosing an airline based on price and/or service expectations to steer far FAR away from United, but if you have upgrades to burn and/or you really like torture - feel free and chose United Airlines.
-jeremy