Perx is good, but also compare to Openskiesonline.com, a similar site.
Perx has a good cancellation policy. There is a $100 fee if it is close to departure time and you cancel, but the rest of the money stays credited to your name for 2 years at Perx, and can be used for travel later. They also have feed back info from customers that have stayed at places so you can hear it from those who have been there. They will help you find a place that suits you personally, so ask questions!
I have also found that using the regular sites, like CheapTickets, Travelocity, Priceline, and Orbitz have given great results, without the non-rev worry.