Background: I am using a macbook pro 15" (2011) with an external Display, which is about three meters away from my macbook. Sadly, the "Apple Magic Trackpad" becomes unreliable when using it with the internal bluetooth adapter of the macbook pro (awkward jumps and jitter) at about 2-3meters already. It changes to the better, when I move the macbook to a different corner of the room, but this does not work for me, as all my other peripherals are sitting in the nasty corner. However, when using an usb bluetooth dongle I get perfect reception all across the room. But, sadly there are a few issues.
When rebooting the computer or unplugging and replugging the usb bluetooth device, the Apple Wireless Bluetooth Keyboard does not reconnect. Usually it does not work, then in some cases it works for a few seconds, and then it claims to be connected (in system preferences) but nothing happens when typing. It can also get stuck while typing, then the last key is repeated in an infinite loop until you press the same key on the macbooks own keyboard.
The only workaround I found is: After reconnecting the usb-dongle (or rebooting):
1. Go to System Preferences / Keyboard
2. Press the keyboards power-button until it turns off (the light goes on when pressing, then press until it turns off)
3. Click on "configure bluetooth keyboard", and instantly afterwards turn the keyboard back on by pressing the button until the light is on
4. Now the menu finds the keyboard. Confirm it and wait until the system stops indicating that it is doing something. Now it should be reconnected and working.
The Magic Trackpad seems to work a little better. Sometimes it reconnects automatically, but I usually click on the trackpad entry's connect-button in the bluetooth tray menu, which works in (almost) all cases.
If you want to pair a bluetooth device, that has already been paired with your internal bluetooth, with a usb bluetooth dongle, you have to unpair it, when the internal bluetooth is active (i.e., when the usb bluetooth dongle is NOT connected). Conversely, when you want to unpair a bluetooth device from the usb bluetooth dongle, the latter has to be plugged in.
I get the same experience with both usb bluetooth dongles I tested. If anybody can recommend a different, where the behaviour is better, please tell me!