I used two (regular and large) of these on a surface interval in Roatan just to try them out. A couple on our dive boat had them. I don't like snorkeling in general but I was actually surprised how well they worked and fit. I have a head that is very difficult to fit for a mask as in 98% of masks won't fit well. I did not use them long enough to have any issues, less than a few minutes each, but was of the opinion that someone had actually built a better mousetrap after all these years. Underwater, their snorkel sealed off completely and you absolutely could not get a breath. As soon as you surfaced, the mechanism cleared and all was good. No clearing involved whatsoever.
Also, I had no trouble taking it off. It was like removing a gas mask, grab at the bottom and lift up. That being said, I was not panicked, struggling for bouayancy, or had a mouthful of saltwater either (likely all three in the accident involved)
Sounds like some meaningful procedureal changs are occurring, but at a high price.
If I'm in the water, I would rather be under water, but based on my experience, I would not be opposed to buying my wife one for surface interval snorkeling. The CO2 buildup is certainly something to consider though.
Jay