Depends how frequently you wish to change.. i always "test" the 6 montly updates.. various places I go are mid-development and it can make a big difference, Bristol city centre for example.
you also can't buy the Speed Camera modded ones - they come in as POIs, but when you search for them the first time, the icons then stay on the screen. They don't beep when you get close or anything, but they are quite obvious as you're driving around.
I have to say, I've made something like 5/6 disks to try and have never had a problem with any.
A very helpful resource for this is:
http://www.x5world.com/navigation-system/
One thing that does interest me whilst im here.. is that the very latest IDrive models are getting hard disks now, instead of DVDs. I would guess then, that the new Z4 will end up with Connected Drive - or Google / GoogleMaps, on it's screen. That kind of thing could tempt me...