MK1 eyeball is so much better and even if you have, like in the aircraft I fly, TCAS 2 fitted when flying VFR you still should spend most of your time heads outside scanning for other traffic.
What I notice is some pilots overly relying, trusting the TCAS and not scanning, to the extend that when ATC calls them about other aircraft saying "we have him on TCAS"
Even with the top of the line TCAS equipment we have fitted the amount of times I've seen one contact suddenly splitting into two, disappearing altogether, not in the same postion when you spotted the other aircraft or when flying near a small GA airfield with aircraft without transponder there will be nothing on TCAS!
Also if you get an RA you (partly) rely on the other pilot flying correctly what their TCAS is telling them, something we train but isn't a given.
IMHO TCAS has its place but not in GA aircraft, because of the reasons listed above together with cost factor.