From my DIY days, I've learned to make sure to buy memory from companies that have a lifetime warranty. Also when selecting the memory you need to understand the CAS spec for the frequency your Mac is tuned to otherwise you could be wasting money or worse yet actually buying slower memory.
Also in the end there's only a handful of memory chip makers and companies OWC, Corsair and Kingston all buy from all of those vendors. The key differences between the memories are how they grade each chip for speed and latency. Very much like what is done in the CPU and GPU markets.
