Originally Posted by supercub
I'm sure nicer components are more responsive, but is it really so significant? Are they more reliable?
Yes, yes they are.
I changed a worn XT front mech for a Deore. Same cables. The Deore was noticeably slower. I've done a similar comparison with X.7 and XT derailleurs.
And the front mech is dead simple. I assume that the difference would be even more pronounced with the more complicated rear.
No significant difference with non-mechanical or non-stressed parts like seatposts and handlebars though, unless you call minor weight reductions "responsive".