I tried the Wal-Mart service earlier in the year, and unless they drastically can improve their delivery times, Netflix doesn't have anything to worry about for quite a while.
I had used Netflix for about 2 years, and generally was decently satisfied with their service, although their constant changing of rates was annoying and off putting.
As an example, if I rented a DVD from NetFlix that had immediate availability, I would recieve the DVD approximately 2 days later. Not a bad turn around.
Contrast that to Wal-Mart, which first, must have a drastically smaller inventory (or not as friendly of a reservation system to new buyers as Netflix) because almost all DVD's had a long wait. On movies that were available now, the EARLIEST I ever recieved a DVD was 5 days after I had ordered.
Now I realize that proximity to distribution centers probobly greatly affects transit time, but for myself the inventory of Netflix coupled with quicker deliver time made me switch back.