I don't have experience with the Xaon products but I have been flying with a Monroy ATD-300 which is a competing product for about 2 years and have enough experience to offer some insights to the usefulness of such devices.
First they require an active transponder before they tell you anything. Second they use pseudo-ranging, meaning the range displayed is based on the strength of the signal recieved and a "mapping" of a "nominal" transponder signal. A strong transponder signal far away, will display closer than it really is, and conversely, a weak signal close by will display further away that it really is. Therefore the range information is subject to quite a bit of error, but is reasonably accurate most of the time but you must understand is is highly dependent on the postional relationships of the aircraft. Altitude indications (the difference from your altitude) are based on the other aircrafts mode-c vs your mode-c, so they tend to be fairly accurate. That said, I found a used to greatly over-estimate how far above the horizon to look for an aircraft. Any plane close enough to your altitude to be a factor will be within about +-2" of the horizon. When I first started using the Monroy, I'd be looking much higher up when the unit indicated a plane at 3 miles +500'. Now I find traffic much faster since I've learned where to concentrate the scan.
A major consideration regarding range is that transponder signals are very much "line of sight". Since the purpose of a transponder is to tell a ground based radar where you are, the antennas are mounted on the bottom of your aircraft. Unless mounted to a permanent antenna under a 337, all of these "transponder detector" units have their antennas on the upper side of the aircraft (on the glareshield), sometimes blocked from above/behind by the wing. This will limit the effectiveness of the unit, but can still increase safety if you are aware of the limitations.
I fly a low winged aircraft and know that there is a blind spot below me both ahead and behind as well as above and behind. Therefore I concentrate more of my scan ahead and below looking for aircraft climbing into me were the "transponder detector" can't see. I cant see behind me, so I still have to worry about faster aircraft overtaking me, but I will bank the airplane back and forth (but maintaining my course) if I get an alarm and can't find the target. Hopefully, that will allow the other pilot to resolve my postion. Never ever, ever make an evasive manuver based on the input from one of these units unless you have made visual contact and determine evasive action is needed. That said, rocking the wings may provide a change in aspect so another pilot can see you but an evasive manuver based on one of these units is just as likely to turn you right into the other guy unless you've found him visually.
All in all I'm pretty happy with my ATD-300 and think it is a good addtion for safety though it is no subsititute for an active visual scan. I've also flown in TIS equipped aircraft and feel the "transponder detector" type units are a good value safety wise though supplementing one with TIS would be the best available solution for small GA aircraft.