Mori55 if you have a NAZA-equipped F450 then you'll already be aware of all the modes, possible pitfalls and benefits so I won't go over old ground! Suffice to say there's a single button you push on the Assistant software to switch over to NAZA mode. It's in the user manual, have a read of the section dealing with the Vision Assistant.
You won't get the full set of NAZA features (no geo-fence, for example) but you will get selectable GPS/ATTI/ATTI or FAILSAFE or MANUAL on S1 and IOC on S2 and the NAZA led signals.
As to how many make the switch? I don't know, but there are probably some who are newcomers to the game who switch too early, and some "old hands" at the Vision (well, as old as you can be after only 4 months) who are perfectly happy flying around in Vision mode and haven't switched.