Like everyone said before, I also use non-Apple RAM for my Apple computers. When it comes down to hard drives, I'm always looking for the best sale which of course doesn't usually come from Apple, but I stick with Western Digital so I know I'm always getting goof quality equipment. I also don't think Apple has the best variety of headphones, thus I have bought earbuds from Klipsch and Altec Lansing.
When it comes to monitor adapters and such I buy Apple brands because I've always had terrible problems with non Apple adapter. Problems that range from being really fuzzy to not even working at all.
Generally I don't go cheap when it comes to things for my computer. I pay a premium for things to work all the time. I can't have things fail in a middle of a major project, it's a waste of money and time. If Apple has that product I'm looking for and it is just as good as any other company's product out there then I will buy from Apple. The price makes up for itself over time. Apple's customer care is definitely a big part too, they have always taken very good care of me.