I have had a bad experience with Amazon, where they cocked my order up. So I had to wait two months for something that was suppose to be there in a fortnight. But I hear that it was an exception and that they are normally pretty reliable.
Yeah, Amazon makes the occasonal fuckup, but for the most part they know what they're doing.
The people selling used on Amazon though, (Used and New from X.XX) I would not trust for anything other than college textbooks. Six dollars for my C++ book, 10 dollars for my Spanish book, etc. Damn cheap.
Also, Froogle is like the most awesome site ever.