I have used one on two dogs, pros are it helps focus a strong dog and it is great for stopping the line getting tangled as you attach the line to the rear ring rather than the collar.
Cons are, a lot of dogs can feel pressured by it, I wouldn't use it on my young female for example. Her corners are super and the line never tangles (touch wood!!) so I don't really need it for that.
I've never used a back ring harness as they weren't trial legal in my sport until relatively recently and to me they just enable pulling and my dogs are fast trackers.