I was consternated about book & hold the other week. I wanted to buy a 1 way ORD-IAD and was getting an incredible price of $80. I wanted to book and hold until the next day when I would know for certain about the trip.

However, I soon realized that this fare required a 5 day advance and had to be purchased by midnight of that day.

Once I did book hold within 48 hours and the effective result was that the ticket was held until departure! (I confirmed this with UA's Dublin reservations.) When I showed up at the counter I found that the flight was several hours delayed, so I abandoned the reservation and walked over to Indy Air and bought a walk-up for a flight leaving ontime within 45 minutes.