This art is Bruce Lee's art, refined from Wing Chun by the addition primarily of fencing footwork for more mobility and much aggression, the inclusion of the backfist(a natural for Wing Chun), and a very strong form of sticking hands that classical Wing Chun students can find irritating and overly aggressive which Bruce learned and adapted from his own sifus in Hong Kong, Wong Shun Leung among them.
Demile adds psychological principles that may or may not be your cup of tea, but one must praise his organization and clear conveyance of Bruce's ideas and the great practicality of the system, which comes in no small part from the rare direct logic inherent in a system that believes utterly in simplicity as opposed to formality and mysticism. Wing Chun, the father style, is an excellent core style that teaches core principles of combat readily adaptable as the student grows, even to other arts, rather than emphasizing an endless litany of techniques that may never apply and may never end.
Demiles' excellent teaching potential I can vouch for, being one generation removed from his teachings myself and two from Bruce. After some small experience in Karate and then gaining a third degree blackbelt in Okazaki Kodenkan Jiu-Jitsu, I came to first classical Wing Chun under Robert Yeung and then Demiles' style of modified Wing Chun in the late 70's, and felt like I was starting to learn all over again. Jiu Jitsu gave me a strong and well-rounded group of core skills, but Wing Chun did the same in a quite different way, emphasizing hand striking and refining natural reactions and timing according to an especially severe and exacting logic and practicality, constantly tested and adjusted in sparring and sticking hands. Learning such different styles at the same time was quite a trick and sometimes felt like a real mind-bender, but it is a testament to the strength and subtlety of Wing Chun Do that the system's value was immediately apparent even to someone who had had plenty of time to build up layers of the kind of chauvinistic prejudice that is the norm rather than the exception between many martial artists, and moreover usually heartily reinforced and encouraged by every new link in a chain of fools thousands of years long.