Wow, I am genuinely shocked by some of the replies on this thread, many of which appear to be complacent as to the limitiations of see and avoid, if not completely deluded. My lookout is excellent (!) but I am fully aware of how inadequate it is - airframe blanking, time inevitably consumed by in-cockpit tasks, inability to look in more than one direction at once, empty-field myopia, lack of relative movement against constant-bearing traffic, etc, mean that it is impossible to reliably detect all possible conflicts. Fact.
Anything that assists you in this critical task has to be taken seriously, such as the economic use of the radio to gain SA on local traffic, for example. I have only just learned about these PCAS devices but I am amazed that there is so much ill-informed comment and reluctance to even try it! The MRX model looks like an absolute no-brainer for the cost - even without azimuth, what a great cue to STOP whatever else you are doing and look out NOW! Even it it only alerts you to squawking traffic, at least you can look for that traffic and, having found it and/or resolved any conflict, continue looking for other traffic with a better mental model of the airspace around you. With good, 3D situation awareness (asking a lot, I know), you can even reslove conflicts without seeing the traffic - you can only have a mid-air at YOUR altitude! As to being an in-cockpit distraction, what rubbish - no more so than the altimeter!
Flying IS inherently dangerous AND it is terribly unforgiving of any carelessness, incapacity or neglect.
...with apologies to Captain A. G. Lamplugh!