Interesting thread. If you do nothing other than switch on your TCAS (or ACAS) then it will do it's intended job and that is issue you with an RA by either presenting you with a Vertical speed command or, in some cases, a pitch command along with an aural command to avoid the conflict in a vertical sense. However with smart use of range controls and by scanning up or below we as pilots (good SOP's help), along with built in features like the transition from other traffic to proximate traffic to TA we can enhance out situational awareness and have a fairly good idea that a possible conflict is coming our way. If VMC we are pointed to the proximity of traffic so that we may be able to sight it. However TCAS is not designed for normal traffic separation in the way GPWS is not designed for normal terrain clearance though, by some of the radio calls I observe, I believe there is a tendancy to get mislead into doing so. I personally had an RA in the LAX area at night in IMC, a light aircraft busted altitude, it was the TA that certainly drew our initial attention out of the myriad of contacts that were displayed. After going through the drama of the following RA, proving the system worked well, an unflapped controller only made minor adjustments to the traffic sequence.