I have an early NT6. Wanted one since I was a lad and bought one in lovely condition a few years back. Its about the same weight as a baby grand piano so its my 'sitting down bass'. On the other hand the build quality is as good as my Fodera and it has a bottom B that is so clear punchy and deep that its almost digital, you could kill a pig with it at 50 yards.
Is it practical, is it good at everything, is it going up in value? no - but its that bass I always wanted in the day and now I can enjoy it.
If you want one do it because you want to.