If you're not booking for the Pope's visit, they usually can get you your first choice, otherwise the competition is hot around holidays, etc and the packages go like lightning. I've saved so much with this site, though, that I make it my business to get in early so I'm all set.
Last trip I saved >50% off airport lot parking, about 10% off the other best price, and got a free shuttle and breakfast. All I need is the best price, and I'm happy.