I think TCAS is the best thing to hit aviation in the last 20 years.
I believe in it more than anything.
However, a few years ago, I was in a beloved Tri-star and we were climbing like a homesick angel only to get a traffic advisory from ATC saying there was another beloved Tri-star at 11,000 FT and we were climbing to 10,000FT. shortly there after we got our RA, but had the other guy in sight , so elected to do nothing more than reduce our rate of climb instead of push it over and go down. We just had an excessive closure rate, and the TCAS picked this up! It is a very rare circumstance where i would ignore an RA! Believe in it unless you have a positive ID on the traffic!