There have been a number of both lab and practical tests. Probably the best one was done by the British Yachting magazine. Another was done by the UK safety authorities after a high speed ferry ran down a yacht with a reflector, having never seen it (http://www.maib.gov.uk/cms_resources/Radar%20reflectors%20report%2Epdf) . The bottom line in these tests is that a passive reflector in the sizes useable on a small yacht may slightly increase your range of detection, though in some circumstances it may actually reduce it.

I know a lot people say "well I was flying mine and so and so saw me" but that is not persuasive unless you break down the detector and disappear from their screen. That is precisely the test that Yachting did. To quote the conclusions in the MAIB test cited above:

"It is recommended that poorly performing radar reflectors are not fitted as it is possible that the user could be lulled into a false sense of security believing that their chances of detection has been enhanced."

Most of the reflectors tested fell into the category of "poorly performing", including the Davis.

I don't say this to dissuade anyone from flying the thing. Just to caution that it is not a panacea for visibility to others. The active transponders test much better, both in the lab and in the field. The newer ones respond to both X and S band. They will set you back the better part of One Large though.