In short, it's not a bad unit - I'd even venture to say it's pretty good. The distances shown are slightly suspect but the general direction of traffic and altitude has been pretty accurate. We bought one between 4 of us for our group aircraft so it was not a serious financial outlay. I'm not sure whether I would have spent £1k on one alone though.
It has shown traffic that I have not picked up with my eyes but it has also not picked up traffic that I have otherwise seen (perhaps these aircraft were not transponder equipped).
Personally, and some may disagree, I think the best traffic avoidance measures are, in this order:
1) The pilot (keeping a good lookout, flying at proper altitudes - although nobody seems to do this in the UK - alternatively flying at strange altitudes such as 3,700 as opposed to 3,500 or 4,000). Fly to the right of roads, tracks, etc. Keep your lights on.
2) Radar services - no reason not to always request at least a traffic service. I usually get this service provided it is not a Sunday.
3) Devices like the Zaon.