Is Amazon EC2 reliable for benchmarking?
No.
Not that this is EC2's fault, but when benchmarking, when comparing A and B, one needs to keep in mind the phrase "all things being equal". (There is a formal name for this, but I forget.) With the EC2 virtualized environment, where you are not in control of anything but your instance, this is just not the case.
Unless you are benchmarking cloud environments ;)
(Edited to be specific to EC2, thanks to knowledgeable comments)