I ran a succession of R1200RTs over the last 12 years and had a Navigator 4 which I eventually replaced with a Navigator 5. They both worked well by the standards of their day and were nicely integrated with the bikes (although not lockable !
The Nav 6 is, no doubt, a very nice piece of kit and some improvement on the earlier iterations, but I've given BMW enough of my hard-earned. I kept my Nav 5 when I traded the RT for the 900XR and it works well still. It now locks to the bike too, so I can leave it on the bike while I take a break. I keep my phone inside my jacket while I'm riding. I don't like the idea of having it on the handlebar - and I can answer phone calls while riding via the Nav 5.
If my Nav became U/S I would look for a good used Nav 5 or 6. For now, I can't see me ever replacing my dedicated satnav with a phone app - I use google maps when I'm on foot and needing directions (with the Nav 5 still locked on the bike !