I love adjustable stems. I just bought two more of them yesterday at the swap meet, for $2 each.
They are heavier, which is why many don't like them. You could make them out of carbon fiber, but I've never seen a CF adjustable stem.
What I like about them is that you can play around with different settings, then once you find what works, you have the option of purchasing a fixed stem at those measurements.
Specialized uses a variation of an adjustable stem on several of their bikes. It is a four position stem that weighs only a little more than a fixed stem. You can set it at -16 or -8, or flip it for +8 or +16 degrees.
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