Reasons, in order:
1) Can book without prepayment required (unlike Agoda). Sometimes need a credit card guarantee but cancellable up to a reasonable time period.
2) Good selection of hotels including a database of more Chinese hotels and motel/hostel places than Agoda database. I tend to be a little more downmarket than the average FT'er.
3) Problems in real-time are dealt with promptly over phone. An example is showing up at a hotel only to find problems making staying there untenable; calling in resulted in instant cancellation, no penalty, no questions asked. Alternate suggestions for substitutes provided in real-time along with instant rebooking.
4) Never showed up and had any booking via elong "lost" or "not received."
5) Often elong all-in prices are better when same facility, same room types area available for head-to-head comparison. And often elong will show availability when Agoda and other booking engines show unavailable.