Used it once for a hotel in Bali. Went fine. One tip is to call the hotel after a few days and make sure they actually have received the reservation from Agoda - we did that and then called again a few days before we got there to make sure they still had it. Same thing you should do with reservations from other third parties like Priceline, Kayak, etc.
Agoda is bigger in other countries. They just haven't had much traction in the US. I think because they tend to have more traction in countries like Australia, you sometimes get the equivalent of the deals places like Disney offer to residents of other countries, that can be better than what US residents get.