I've ridden with a couple of people using rear-facing radar. It seems OK and not too intrusive, but I think it doesn't distinguish cars from other cyclists and it doesn't often hear things earlier than someone with good hearing. Maybe a good addition to a mirror if you find it difficult to look behind you.
I built a singlespeed bike (with freehub, not fixed) out of old bits last autumn. I did it just for fun and thought I might use it for the odd trip to the shops or nipping down the valley to visit friends. In fact, I found that I enjoy riding it much more than I expected to and have done 1,273 miles on it since 27th November, including metric centuries, imperial centuries and even one '200' (km).
I have used my ancient Etrex GPS for 11 years, on over 30,000 km of rides in all sorts of bad weather and it hasn't failed me once! There was just one time when it struggled to get a satellite lock and a reset soon sorted that out.
