TCAS was designed by Honeywell (I think) for use primarily in the controlled/regulated airspace environment. Obviously it works just as well away from that but is limited in its algortihms (IP of the maufacturer) in its reactions to conflicts. Can't say I've ever seen an RA from high RoC/D, but have seen quite a few TA's.
Even large ac operators (ie airlines etc) should maintain VS within 500-1500fpm within 1000' of levelling off to prevent a false advisory to ac above and below. I'll stand to be corrected on those figures - has been a while!
The upshot is that TCAS is very useful in the open FIR - but only as another tool in the box - no excuses for not looking out of the window (damn that's the qfi in me)
However, having flown with it, you never want to go back to the old days. Also of interest however is the usage of GPS in accuracy of navigation - ie being so good, we all fly in exactly the same bit of airspace rather than the slight random scatter VOR and the old INS's gave.