I just bought $200 worth of BumGenius diapers. It was an investment, so it will force me to be committed to cloth diapering this go around.
For those going with sposies, I agree you should wait until you see what fits your baby best, but if you are a couponer and find a great deal, I wouldn't pass it up, either. Pampers NB are really versatile and amazing.
Stock piling diapers can be risky, but I know people who have saved thousands of dollars that way. You can always exchange diaper sizes (though not brands) at the store you purchased them at.