Too many problems with third party ram. Some works for awhile and then another OS update comes along and knocks it out. Read the Mac forums.
I like to have a unit that will do what I need it to do out of the box without building the damn thing myself, and don't mind the higher cost IF IT WILL DO THAT. Same reason I'm angry Apple doesn't have a blu-ray burner and blu-ray authoring support worked up yet and let PC's and Wndoze take the lead.
Same reason I'm considering PsyStar, because they'll build something to my specs that WILL do what I need it to do out of the box. For one-third the cost.
I'd rather buy Apple, but if they continue to crap on my needs, then my money goes elsewhere.