I also have two Canon 580 EX I use now much less (and only when I need TTL function in action photo).
Why do my YN560-III? To make images per trigger radio. This model directly incorporates a radio receiver (super convenient) I active with a radio trigger (YN 603) mounted on my Canon bodies (The 603 can trigger receptor but also mounted under any flash. Besides, I highly recommend, it is a small wonder.). I use manual mode flash units on legs. In this type of use, I found the YN-560 III excellent. They quickly recycle. It is not lost in a complicated interface. The transmission link is reliable over a long distance (in practice I find it much more flexible than the IR). With my Canon flash, I'm always a little alert, anxious, in fear of an unforeseen, a poorly tuned trick of setting unexpectedly sleep ... With my Yongnuo and radio trigger, I am much more relaxed, more serene to concentrate on shooting. I still carry my Canon IR TTL trigger (ST-E2) but it's been about a year since I'm using it more.
You just measure the limits of the tool: no TTL (there are others Yongnuo for that) and no high speed sync (at least I think I did not search the documentation.).
Its very affordable price (one-tenth that of a Canon) allows buy multiple (to increase the light output outdoors for example). The low price is also a parameter when installing the flash outdoors on a non-weighted tripod with an umbrella (classical situation in the studio or improvised with strong time constraints) there are significant risks of falls when it there is a little wind. If the flash falls and breaks, it is the case of 60 euros, not 600.
Also note the arrival of YN560-TX, a small extra box (40 euros), which allows to remotely manage and easily a group of three flashes. We put three tripods. We light flashes and adjusting the light without having to move constantly during the session. In short: a mobile studio of excellent quality and reliable for a very low price.
I do not spend my time comparing catalogs, so am not very familiar with all the news, but I find that the Yongnuo approach (oriented system manual flash and another oriented system TTL flash) is justified fully.
