I know nothing about TIS or TCAS (and little about anything else, many would say) but there is another anti-collison unit which depends on conflicting aircraft having transponders. See:
http://www.avionix.com/collis.html
While I am not an advocate of making transponders mandatory, I think we may have to get them by 2005-8 whether we like it or not. If so, then this 6-800 dollar kit may be a sensible add-on - if only we have the room and weight capacity to install it and batteries to run it, which I don't.
Chris N.