Ok, let me tell you my experience and then you can decide what to do.
I have a new MacBook Pro - I found a source on ebay that provides genuine Samsung RAM for these Macs and at the time it was £55 while Crucial were charging nearly £100.
I purchased the ebay RAM and after a day or so of use I started to get kernel panics and system freezes within an hour of turning on my Mac.
Having read about certain MBPs suffering this problem even with official Apple RAM I sent my computer off to be checked under AppleCare. They took ten days to inform me that there was no issue with the system. I was suspicious of their findings given that each RAM stick on its own caused me no problem.
Anyway, I decided to take the plunge and ordered another set of 4gb from Crucial on a Monday. They delivered the very next day (under the free delivery option!) and I have had that RAM in my system for two weeks now and not suffered a single freeze.
So, I fully understand the desire to save £20 or so but you'd be wisest to learn from other people's mistakes and go for Crucial - the customer service is excellent and, if they're now only charging £50, the price is very competitive as it is.