As macman said, the Apple unit is usually enough. Unless you're really concerned about another KB's ergonomics, or there are specific extra function keys that you need that the Apple unit does not have, you'll probably be quite happy with the Apple unit. Plus it will match your computer.
The apple kybd is fine --- I'm still adapting to it --- biggest thing is that it has 2 built in usb ports which are very handy -- esp in a mini with limited ports
I wouls imagine it is power consumption issues. Since many USB devices draw some prower from the USB port, the 4AA batteries in the wireless KB probably wouldn't last very long.
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