I am with IO540 on this
I too fly with TCAS, and in the controlled airspace environment of airways flying it is perfect. In principle, everybody has a transponder, and TCAS provides both situational awareness clues as well as a degree of protection if used correctly.
Outside controlled airspace, there are too many non-squawking aircraft, Mode A targets and other bits and pieces floating around for it to be as effective. I switch to TA-only mode under those circumstances so that at least I get some clues as to where traffic might be visually acquired. More than that is deeply unreliable.
The TPAS units I have seen advertised look like a monumental waste of money. I played with one in a friends aircraft, and apart from inducing panic that an aircraft might be close, it did little to help.
My experience was that it had big difficulties spotting the difference between traffic at the same level half a mile ahead, and traffic over the top of us 2000+ feet above.
2D