All, Thanks for these replies it is showing some interesting themes...
I think the main point that is coming out is that these are all equipment and should be used as aids and information sources to assist the pilot - but do not replace a good lookout.
Also I think comment regarding FLARM in the alps raises an interesting issue. In the Alps people are flying closer together, there have been more incidents of aircraft occupying the same bit of airspace at the same time and therefore people choose to install this technology. But because of the terrain etc. more aircraft carry FLARM that don't therefore the solution works. However there is no mandate to carry it, it is purely an example of how the flying population recognise a danger and take their own approach to do it.
In the 'congested' skies over the UK, (I put that term in inverted commas because can you argue that the UK skies are as congested as a valley in the Alps?) there is, maybe, not the collision risk that is present in the Alps and therefore people feel that they get no benefit from installation in skies such as the UK.
Please keep the replies coming... it is good stuff.
I think there is enough for me to throw my 2 pence in the ring: -
1) Do you know what it is? - Yes
2) Do you have it? - No
3) Would you buy/fit it? - No
4) If so, why, if not, why not.
I think there are other alternatives already available that if used correctly can help and have minimal cost, for example actually using the ATC services available, ensuring the aircraft is flown properly (e.g. in trim etc) allows more time for look out, taking a friend flying and asking them to assist with the lookout (also alleviates boredom), thinking about the altitude and where you are flying (everyone fliew at 2,500 and 3,000 feet for example, so fly at 2750; if you are above the transition altitude use the correct FL even if VFR and if you are flying along a line feature, even if you are not using that feature to navigate recognise the fact that other pilots will... fly on the correct side etc. Doing your downwind checks before you fly the circuit, then all you need to do is look out while you're in the circuit (Don;t forget the Reds, Greens, Blues on Final tho)
Anyway, my 2 pence worth!
As I said keep 'em coming this is a really good debate!