A salutory lesson in how little we actually see despite keeping a sharp look out can be had by flying a machine with TCAS in the open FIR. Despite 'knowing' (as in bearing and distance) where to look you it can be a devil's job to spot the bogey!.
Quite so. On balance I like having the Avidyne tell me what's out there, but sometimes I feel that the former blissful ignorance makes for more relaxing trips!
It is surprising and worrying how many aircraft you can't see even when TCAS is showing you just where to look.