For the price it's seriously tempting, because while I'll an avid supporter of the sayings 'you get what you pay for' and 'buy cheap, buy twice', Canon's offering (MR-14EX2) is simply ridiculous at £500, and I'm finding it difficult to justify the expense for the odd macro shot I may do (their mk1 model was around the £300 mark when it was still being sold, and so this price hike for the mk2 also irks me).
Oh yes, I'm a long time fan of Yongnuo. As well as Macro lights, I also use 3 Yongnuo 600ex-rt and 2 686ex-rt along with the Yongnuo YN-E3-RT (the 686 rt is new on the market and utilizes a long life lithium battery instead of AA's)
I had the Canon one on my older 100 macro, but when I changed lens's to the 100 L, I needed a 67mm adapter - Canon wanted £35 for one, so I sold the mr14ex and bought the Yongnuo variant - it came with all needed adapters. I've now sold that and use the Yongnuo YN24ex, a lot more versatile, but twice the price. The yongnuo works just as well the the more expensive Canon.
Only downside to macro light is the harsh lighting they produce (sure you knew that anyway), so you do need to "invent" some sort of diffuser for it. I used some old packing foam that came protecting the screen of a new TV I bought, along with some strips of silver reflective cardboard and the ubiquitous gaffer tape, worked a treat. A quick google will get you plenty of hits
I've got loads of flashguns (2x Canon EX-550, 7x Yongnuo manual flashes and a Profoto 'fashion' ringflash) but I've long wanted a TTL macro ringflash just as a simple 'grab and go' solution if I see the occasional exotic bug in the garden or something (nearly got a photo of a dragonfly sleeping the other day)