I've used booking.com frequently for the reason mentioned above - cancelable reservations. And as atleast10hats noted, I've used them for backup reservations that I later cancel if I've been able to beat them. Recently, though, I used them for a small property in Istanbul. I made one in Vienna, but found a better deal almost immediately after at the hotel's own website (the hotel offered a "single occupancy double" rate that booking.com didn't.) Booking.com is very clear about the rooms offered, including size in sq. feet or meters and included amenities and other costs (including wi-fi) So it's often one of the first places I check.