Just like booking any hotel... decide what you're looking for. Value, luxury, or both. Used perx in Cancun w/ no problems. Great all inclusive rate.

 

The girlfriend and I have used perx several times for all-inclusive vacations and every time have had a great experience. A lot of their deals with hotels include "bump protection" where if you can't get to your destination they hold the unused credit for 1 year and you can use it to book any other resort at any time throughout that year. Alternatively, they can just slide or change your reservation a day or two with no penalty. Have even had to make a last last last minute change with them (like I'm sitting in the back of the plane on a Sunday morning hiding the cell because the door has closed and the FAs are barking at me to turn it off) and it was all sorted out by the time we got to the hotel.

Downsides? Have heard of occasions where calling the hotel directly can get you a better deal. Also, like another poster mentioned, I don't rely on their info and ratings but instead do a lot of research on tripadvisor.com (greatest travel website EVER).