Works satisfactorily once I got it paired and the keymap set up as per instructions found here.
Now the kb pairs up as soon as I switch on, then in an xterm I login as root and set the keymap which takes all of 20seconds.
Very occasionally the Enter key stops working and I have to use Ctrl+Enter instead. Probably some confusion between the BT keyboard and latching keys on the N900 kb, but to be honest it's a fairly minor thing that I can't raise the enthusiasm to try and fix

edt: Just to add...I've also got my BT mouse working now which means my N900 is a full on laptop replacement......fantastic bit of kit this phone