Before you had to know your airline and aircraft type to look at the graphic showing which seats were good and which were bad. Now you enter the airline, flight number and date of the flight, and the site finds out which aircraft type will be used for you.
It is even clever enough to deal with airlines that have multiple configurations of the same aircraft type by bringing them all back so you can view any one of them and flip back and forth to compare. For example Qantas has 2 different configurations of 767 and the search brings back both configurations if the flight is operated by 767.
If the search is unable to locate the aircraft type (sometimes airlines do not supply this information) a page with links to all the airlines and aircraft in the database is shown for manual selection.
It handles multi-segment flights okay by asking you to select which leg you are flying. Unfortunately if you are flying more than one of the legs you have to repeat the search for each leg. Perhaps an option could be added to show all the legs?
The search isn't yet foolproof. It doesn't appear to work on codeshares - you need to enter the operating airline and the operating airline's flight number.
The seat diagrams are not perfect - many airlines are missing and some configurations are missing or out of date (eg Air New Zealand's space+ seating on A320 and 767-300 aircraft are not shown). However, the new search function does make SeatExpert accessible to many more people and thus is a good thing in my view.