I've been to the one in Anaheim, CA (big warehouse) about 10 different times and the one in Culver City, CA once (smaller venue - more like a medium sized store with warehouse shelving).
FYI: These sales are books from the "Scholastic Book Fairs" not the fliers that come home with the kids. Many of the books are the same or similar but they're not the same division of scholastic.
I've always found the sales more than worth my time and the drive. I have always had use for my $10 off $50 coupon and then some (I rarely get out of there under $100 - we really like books in this house and they also make great gifts).
There is always a bargain area, another area where things are at set prices (50 cents, $1, $2, etc) and then the rest of the warehouse is just shelves and the books are 50% off the price listed on the cover. It's a great deal!