Mine's arrived now. First impressions: Feels and looks very premium. Good solid weight. In general it feels like the Magic Keyboard + 5% of size and weight. It's not at all wobbly, and the keys have a bit more travel and bounces back with a nice spring. The key coating is not as nice, with a bit more grip. If you drag your fingers across the MK it's all smooth, if you drag your fingers across the X1 it squeaks from the extra friction. Not sure how this will play out in real life, but as I'm typing this it's not bothersome.
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The keys are about 5% larger as mentioned with less gaps inbetween. The keyboard is pretty much exactly as tall as the MK, but with the top row being half height. F3 and F4 defaults to Spotlight and the tab switcher instead of Exposé and Launcher, which is a wash for me. Spotlight and Exposé as defaults would have been ideal.
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The key caps are a heavier weight than the MK, but still feels very Mac like. This is 100% a Mac keyboard for Mac users.
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The backlight is ....ok. It gets nice and bright and is much better to deal with in the dark of course. The light looks a bit cheap, with some uneven bits especially under the space bar. The temperature is very cool, and it has a "kiosk" quality to it. It's serviceable, but feels like the least premium aspect of this product.
All in all perfectly good value, plus the black and space grey now matches my black Magic Mouse.