Concur with TC & ST - TCAS is excellent but it's not perfect and is only in addition to normal good lookout practice.
It's sometimes more of an "area" tool which tends (in my experience) to give an accurate distance readout but often inaccurate bearing, anything up to 90deg out sometimes - possibly due to the 20+ other aerials and metalwork hanging off the aircraft? When ATC pass traffic info, it can be useful to tie a known contact into TCAS, especially if you can see it - but important not to forget that, unless you are visual, there may well be others out there to confuse.
It's interesting to see how many contacts suddenly appear when ATC info is passed & we reply "thanks, looking, nothing on on TCAS at the moment"!
As already mentioned, it does need the other chap to transpond, of course.