I try to book through the airline's individual site for the booking bonuses; however, I have found ITN's display of fare classes VERY useful (especially for upgrades, as stated above! It's nice to be able to "count" down as each elite level clears). The disadvantage of ITN is that it does displays neither scheduled flying time nor meal service, something some other sites (such as Expedia) do.

I have used Preview Travel a few times due to promotions, but the site is rather slow and its reservations system needs some updating. It still does not recognize the new Shanghai PuDong (PVG) airport, and as some carriers (such as UA and NW) have now moved operations from SHA, Preview Travel seems to think that no flights are available.