I have no experience using any mosquito killing or repelling device other than 'Off', however I read somewhere that these devices attract mosquitos by emitting carbon monoxide (or is it carbon dioxide?) I will say that my anecdotal evidence is that the emission of carbon monoxide does indeed attract mosqitos, as my riding lawnmower seems to attract them big time.
So my questions based on that would be
a) From how far away are mosquitos attracted to (xyz)? If it's from hundreds of feet away, it would seem that the attraction to the device would bring as many mosquitos into your own proximity as into the range of the killing device.
b) Have you got the device set up in such a way as that you hazarding your own health from carbon monoxide poisoning?
I could see such a device working if it is set up as a 'lure' that attracts the mosquitos away from people and towards the device, and doesn't attract more mosqitos from out of the area than it attracts away from people. It seems like this would be much more effective without being hazardous in an area that has excellent ventilation with a limited supply of mosquitos, say a giant tent that has screening that allows free flow of air, yet only allows a few thousand mosquitos in at any given time through entrances that are constantly being opened and closed.