I've found that the site is not always accurate, but it has worked for me many times. It's helpful in figuring out when shipments come in. Even if the inventory is wrong I have found that if I show up early enough when they bring it out to stock the shelves, I can usually find something I need.
I've found the site to be very accurate for me as well. My local walmart only seems to get shipments of ammo in once a week...I check the site every morning for 9mm ammo. When the site shows they ahve stock, I show up at their stocking time and they have always been putting ammo out. Just a matter of figuring out when your walmart stocks the ammo case. Just calling in may not suffice if its still in the back and the clerk just checks the case.