I'm in the minority, but I say not to buy on layaway. I've thought layaway is a good idea, but I've read some advice from financial experts saying it's not. For one thing, some places still charge fees for the service. Also, and this is a big one for me, they don't honor sale prices which usually come later in the year. If you're doing layaway to make payments, you're arguably better off using a rewards credit card, but I think it's best to just make a Christmas fund and save little by little over time.
As for the issue of items being out of stock, I think that's less an issue today than it used to be. Stores are able to stock more quickly, and the cyber shopping gives you a seemingly infinite number of shopping options.