I'll go against conventional wisdom with a little story. I had put RAM bought from a very reputable website (very commonly stated on macrumors) into my Powerbook about 3 years ago. About 6 months after placing this RAM into my PB, the logic board pooped out. I had Apple Care so I brought it to the Genius Bar. They ended up replacing the LB, but told me that the failure was due to the 3rd part RAM. They then told me I was "lucky" they covered the warranty due to me placing the RAM in. I then bought Apple RAM and have had no problems since.
Was the LB failure due to the 3rd party RAM? I have no idea.
But, I do find it kind of interesting that there are all these people complaining about how Leopard has caused them all these problems and I have had zero problems with the upgrade. Is it the RAM? I have no idea.
Never bought Apple RAM, never had problems. Anecdotal evidence really doesn't prove anything.
So, how did the Genius determine (and more importantly, prove) that the RAM was at fault? Particularly after six months of operation? And how exactly is RAM supposed to catastrophically fail another component (once again, after six months)? If RAM's bad, you replace it--the logic board would be fine. The Genius was blowing smoke, that's all.