This won’t work on many US Airways flights since they are always full. I knew I was taking a risk with a long haul United flight as well, but decided there is nothing to lose and an empty middle to gain.
The trick is that since most people only want a window or an aisle they will tend to not choose a middle seat if they don’t have to.
My husband and I were flying in Economy Plus on a Boeing 747 from LAX to SYD. The 747 has a 3-4-3 configuration which means if I take the window and he takes the aisle the odds are that unless the flight is full no one will choose the middle. That worked out and we had an open middle for the whole 14 hour flight.
What happens if someone actually takes the middle seat? Depends on how much you love your seat versus who you are traveling with. Many suggest you should offer the person the window or the aisle. Since your are being nice anyway you could just give them the one you prefer less. If you don’t really care about your travel companion you could also just keep your seats and have a stranger between you.