I guess I've been pretty lucky. I've bought some cheap stuff that turned out to be crap, and some expensive stuff that turned out to be not as great as hoped, but nothing expensive yet that turned out to be terrible. Least bang for big bucks, performance-wise, was race wheels. But to be honest, I got them because they looked awesome on the bike; the notion that they'd improve performance or ride better somehow was just the "practical" reason that allowed the purchase.