Really, really vast selection of books. Some at eye-poppingly low prices.
Now, just be aware that these are, in many cases, ex-library books (not a fact they seek to conceal, it's stated clearly there on the site somewhere.) So be prepared for stickers / stamps / card pockets / chips / dings / dog-ears / minor tears / blemishes / yellowing, etc. but in many cases, the condition of the book inside is perfect. Bear in mind these are priced well below market value, so it's well worth it.
A massive plus is free shipping worldwide - that in itself makes the purchase worthwhile to me. I've often paid more in shipping from Amazon than the price of the book itself!
Two tiny things - no online tracking for shipments, in many cases (though everything I've ordered has arrived promptly, and without problems). Also, clicking on a book often yields a "darn, we just sold that"-type message. Maybe the stock turnover is actually high, so they can't update in real time.
Big plus - your purchases do a bit of good for world literacy!