Online stores sell DDR3 memory from companies like Komputerbay for a fraction of the price that Kingston, Samsung, Crucial, OWC, etc. charge (not mentioning Apple OEM).
Most reviews are good, but some people report it not working. That's not actually a problem, since Amazon will give you a refund; so the important question is:
- With RAM, if it works initially and passes hardware tests, does that mean it's likely that it won't have future problems?
(Reviews only represent a fraction of the people that bought the items, so if it works right away they're likely to leave a positive one and not come back to edit it if things go wrong months or years down the line.)