Wal-Mart rocks if you like long lines and inferior YumCha brand merchandise.
I went to Wal-mart to buy some a few air cleaners for the house a few years ago. A quick look at the merchandise convinced me to head over to Target for some quality products. They cost more, but they last much longer. I'll pay for quality anyday.
And as for Costco... I just don't care. As long as they continue to offer good merchandise at decent prices, they'll get my business. If they want to throw their money away, let them. I'm not going to throw mine away.