If you go Bluetooth, get a good class-1 transmitter. The ones included with most keyboard/mouse sets seem to be class-2 at best. In theory the class-2 range of 10m is plenty, but I have had problems with lag on the mouse as near as 5-10 FEET from the transmitter (with Logitech -- I stopped...
My experience matches Oregon's. I replaced the master device with a Linksys USBBT100 and removed all Logitech software, re-added the kb and mouse devices, and have had flawless operation since. This was the MX5000 kb/mouse set.
I also removed Setpoint. I lost a few frilly button features...
Logitech's dongle seems to be very low power. I had to replace every one of them with a class-1 transmitter, even for those used in the 5-10 foot range. I suspect it of being a class-3 device, though I can't find any proof either way.
In short -- Logitech's has many flaws. :)
I had horrible lag with the mx5000 set, until I replaced the inclouded Bluetooth dongle with a Linksys USBBT100. That's a class-1 device, and I suspect the included one is class-2 or worse. I went from horrible lag at 5 feet, to no lag at 30'. (My typical use is 15' or so, but I got excited...