I used it when moving to an area with which I was wholly unfamiliar. That is, it didn't really matter that they assigned a realtor to me, because I didn't have any in mind, nor did I know anyone who might refer me to a good agent. It was okay, and getting a few thousand back at the end of the process was a nice cherry.
That said, I highly doubt I'll ever do it again. After having been through a few real estate transactions, the realtor can make or break you. It's something that you want to choose on your own - not leave up to USAA or anyone else.
Also, the program gets its money by basically requiring a kick-back from the agent. Accordingly, many higher quality realtors, who aren't hurting for clients or money, just don't accept referrals from USAA. That's not to say that you will necessarily be assigned a bad agent, but you get the idea.