I just put the ambi unit from Bushmaster on my M4. It's really poor quality, made from soft material. Out of the package it wouldn't latch, I had to file the hook to make it match the GI latch.
As a lefty I can operate it with my right hand in the GI configuration, but I was looking for something better. I found that operating the ambi unit as designed puts a lot of lateral pressure on the handle (as someone above mentioned) and can cause it to bind some. The "pinch" position necessary between thumb and forefinger is not a very strong hand position and took a lot of effort. I was also scraping my fingers on the edge of the collapsible stock.
After working with it for a while, I discovered that it worked great if I simply cupped my right hand over the top of the upper and pressed backward with the edge of my index finger (backed up by all my other fingers). The large part of the latch on the left side gave plenty of purchase and the blade shape of my hand in that position allowed a powerful and positive movement. This method seemed to eliminate a lot of the sideways pressure on the handle. It also prevented scaping my hand on the stock.
Now that I've practiced this "cupping" technique, I think I'm going to get and try the other, non-ambi tac-latch, the one with the square shaped handle. This should be more comfortable than the knife edge of the flat ambi unit. I might even plasti-dip the handle and see how that wears. As a lefty I don't have to worry about being stabbed in the back when slung, and my simple (aka Burnsed) sling, being on the lefty side, is also out of snagging range. We'll have to see what else it snags on, though.
AA