I cant help feeling there is some lack of understanding of TCAS in light aircraft.
I would like to set out my experience.
The last few days round trip was to and from the west country - a couple of hundred miles in each direction.
About 80% of the flight was with a RIS, 100% with TCAS on the MFD. The outbound route was all low level - not above 3,500 feet and coming back about half above FL75 and half low level.
TCAS was set to display all traffic - the options are above and below with or without a height buffer and unlimited. The traffic appears as the usual diamond on the moving map with altitude if the traffic is transponding mode C or S and an arrow to indicate the altitude trend.
Over the total route there was one primary contact identified by AT which was not transponding but presented a "threat". Of course it was not displayed by TCAS. The position and altitude of all other traffic, and there was a lot outbound, coincided with that reported by AT.
At GA speeds, (and in this case a cruising speed of around 175 knots) even with the traffic closing from ahead, it is simple to adjust the track and height to "ensure" separation. Moreover in almost every case it is reasonably straightforward to "work out" if the traffic is en route or manoeuvring.
The system may not be perfect but with the autopilot engaged and TCAS turned on your situational awareness of other aircraft and your ability to give them a wide berth is a million miles apart from working the same task visually. Moreover it enables your eyes to be outside hoping to pick up anything the "automatics" have missed.
What’s my point. Well TCAS is not a panacea anymore than FLARM. However both systems provide a very significant improvement on the mark 1 eyeball. Most en route traffic above 3,000 or 4,000 (on this trip at any rate) would seem to have been transponding and TCAS appeared to identify the traffic accurately 100% of the time. For me I am a great deal more comfortable having TCAS in the cockpit. I realise it is not a guarantee that I wont have a close encounter but it stacks the odds by another significant margin in my favour.
I felt I should make this post because I am not sure from some of the other posters whether they write from the perspective of having used TCAS in the cockpit. I recognise that I am privileged to fly an aircraft fitted with this system however as the price of modern avionics is driven down, glass cockpits become more common and less expensive alternatives become available (such as PCAS which I have found to be very reliable and already with the ability to link to a moving map display) I think we should not under estimate the safety enhancement these systems bring to general aviation. I made the decision some while back never to fly without a least a PCAS - it cost me a few hours flying but I reckon it was one of my best buys.
Finally, as I said before, I sense there is that brigade who are all too ready to chip away at TCAS or PCAS by inferring it is yet another excuse to fly the aircraft on instruments in VMC. If you allow it to be, then I guess they would be right. However, I flown with plenty of pilots who have a cockpit of steam gauges without a GPS in sight that fly on instruments in VMC. TCAS and PCAS do not encourage you to be head down any more than any other instrument and used properly they free up time to maintain an effective scan.
Sure, as another poster commented, there are plenty of aircraft not transponding but look at it this way, if I can identify all those that are, at least I have narrowed the odds in my favour. Maybe by a bigger margin that you might think because if it is fast its probably transponding and I reckon anything I am closing at 400 knots or more I am not going to do a very good job of spotting.