Saving 1 lb or so is a bad deal if the stuff doesn't work well.

In the 90's I had a Schwinn homegrown usa, a bike that attempted to use all American made chi-chi parts. It was light and looked great. Problem is it didn't work. All those CNCed parts just sucked. I raced on the bike, but...
In private I trained on a simple ti frame with all XTR parts. It weighed about 1/2 lb more, but worked perfectly. It shifted flawlessly.
It seems to me that bikes havn't changed much since then. The materials have slightly improved... I suspect mid level pros do the same thing these days.

My advice is stick on campy/dura ace/XTR and ride the hell out of it. On hilly days ride light wheels and on flat days stick on aero wheels.

......However there are still some super light weight parts that do work perfectly.

I would go further as to say ride full groupsets where possible and ride the hell out of them, be they Deore or XTR, Xenon or Record, Sora or Dura Ace. Just ride the hell out of whatever you have!

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http://worksrider.wordpress.com/

There are plenty of light weight parts that work great and compromise nothing but price. There are compromises worth making and there are, of course, compromises that aren't worth making. Mountain bikes in particular demand compromises but we're all making them, whether we're weight weenies or not. I think sacrificing eight inches of fork travel is worth it in order to make climbing a possibility that most of us are willing to make. Then again, I have a friend who is riding a 47 pound RMX with an 888 on it as his normal, everyday bike (sadly, he breaks more parts on that bike in a year than I do, but that just says something about the way he rides).

Most of my friends consider me a true weight weenie, but even when building a new super WW bike, I made sacrifices to maintain good quality function. That means heavier tyres, a more plush fork, and compromising some weight savings for increased strength in the wheels. XT will almost always be stronger than XTR, but XTR is adequate for nearly everything. I draw the line at sacrificing ride quality in my fork to save another 200g, but that decision was based on the courses I race on most often. If I lived in less technical terrain, perhaps the sacrifice would be worth it.

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2001 Bianchi SL-2 Reparto Corse
2006 Rocky Mountain ETSX 70
2006 Scott Genius RC-LTD
2007 Rocky Mountain Element 70

this is always a good thing to think about.

we want fast, not light per se

pushing or carying a bike doesn't sound fast to me

I also think that being able to depend on your bike, ride in comfort and without breakdowns is most important. Always keeping the weight in the back of my mind, but strength first. So you won't see any dremelwork or alu screws on my bike. Stronglight, is my motto

In 20 years of mountain biking I have found that simplicity offers highest reliability, so I have thrown away all the "useless" stuff like suspensions and gears (ebay often being the dumpster). As a side-effect it has lightened my bike and let me re-discover why I like to ride. Weight weenyism by making different sacrifices.

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blogging at 29in.ch