I have & have used an avalung quite a bit.
In snowboarding, I would put it in my mouth before each run & make sure it stayed there so I could grab on to it, regularly I would re grab it mid run for practice & it worked relatively well. (open face helmet btw)
As I started running higher risk lines on my sled I attempted to do the same thing with it, not nearly as smooth, you would have to take a hand off the bars to get it where you wanted it. I don't mean a quick jerk like pulling a bag, it was a bit of a process, move away face mask, scoot up helmet, push in the mouthpiece.
I'm pretty anal about avy safety, I'm stupid enough & willing to accept a level of risk that most aren't, so I take any & all precautions that I feel may increase the likelihood of a situation going my way (I don't take more risk because of the gear & training, it's the other way around).
In my attempts to use the avalung effectively, I came to the conclusion that it was VERY unlikely to help me. I just sold mine tonight after it sitting around the house for a year or so.