I've been using the MX900 for about 5 days now on an XPS using the internal Bluetooth adapter. It's been working quite well with a couple of exceptions:
1. After a complete shutdown/restart of the system, about 50% of the time it is not immediately recognized. I have to do a search of bluetooth products and push the connect button on the bottom of the mouse to put it in "discoverable mode", then instuct the computer to connect to it. It takes about 10 to 20 seconds to do all this.
2. The task switch button (second button down from the wheel) is not usable.
All other buttons seem to work as advertised, however it looks as though you cannot customize the buttons to anything other than it's default settings. To do this, you have to attach and use the base station and use the Logitech software. Not a problem for me as I'm happy with and prefer the defaults anyways.