It’s official. Apple, we have a problem.
I have now owned three separate Magic Mouses (Mice?). In each case, I have had the exact same symptom. The Mouse’s Bluetooth connection to my 2009 Mac Pro kept cutting in and out. It would happen so often that I was ultimately unable to use the device (which is why I went through three of them).
Of note, I also have an older Bluetooth Mighty Mouse, which has worked and continues to work perfectly (as I previously covered in a Macworld article). This led me to conclude that the problem is not simply with the Mac Pro, as some have suggested.
I have now done further testing. With my third mouse, I gave up on using it with my Mac Pro and connected it to my MacBook Pro instead. For the past week, it too has worked perfectly — not one disconnect. For me, this confirms that the problem is due to some odd interaction between the Magic Mouse and (at least some) Mac Pros. Neither device by itself causes the symptom. The two simply do not play well together.
Hopefully, Apple is aware of this and will issue some sort of firmware update that fixes it.