When you buy from a place like DataMem, you're paying extra for a brand name, the guarantee that it will work, any replacement costs after the sale, and a lifetime warranty. Personally, I went with DataMem when I bought memory for my PowerBook, and my MacBook. The reputation that DataMem carried with them, as well as the lifetime guarantee. I based my choice on the facts that Apple's hardware is notoriously picky when it comes to types of RAM, and that I'd rather be protected if anything went wrong in the future.
I'm not saying that the RAM itself is any better than the other vendors', but in my case, I would rather spend an extra $40 or so for the security in knowing that I'm protected if anything goes wrong in the future. I'm sure there are plenty of people who have purchased value-priced RAM and have been completely happy with it. However, I'd still say that if you don't mind spending the extra money, it's probably worth it to go with a vendor like DataMem.