Not tried one but seen the advert, browsed their website.
They seem to work by listening out for transponders replying to third party interogation (ATC or TCAS). The strength of the signal received is used to estimate the distance, and the altitude is available if that aircaft is squawking mode C. Since there's no other position information in a transponder transmission, it has no idea of azimuth. It can estimate closing rates based on the rate of increase in signal strength, and can track multiple signals (aircraft). Or it least it can if they are squawking different codes. I don't know how/if it can differentiate between multiple aircraft squawking 7000...
Might be more useful when we all forced to have mode S?