We had heard about this site for a while, but hadn't bothered exploring it. However, since we were going to LA for the LA Times Travel Show, and wanted to upgrade our coach fare to business class, now was the time.
Expert Flyer does a lot of things, but one of the things it does best is to make it very easy for you to find out exactly which flights have seats open for frequent flyer mile bookings or upgrades, including waitlist information, if available. If nothing is available for your flight, they'll send you an alert if something does pop up.
So in a few minutes, we knew exactly which flights to book on our dates of travel in order to upgrade our $399 coach fares on United from New York/LA to business class. (Recently, United decided to eliminate access to their upgrade inventory, but ExpertFlyer hopes to get them back on board).
They also offer access to real time seat maps on over 100 airlines (of course, our friends at Seatguru.com show seat maps, but not which ones are empty right now on various flights). And they have some neat published fare info with rules, restrictions, and lowest fares (this is sort of like EasySabre, for those of you who remember that product before it became Travelocity).
They offer a free 5 day trial, a basic plan for $4.99 per month or a premium one (which unlimited queries) for $9.99 per month; and there are also annual plans. We found it a useful tool.