I have used one fairly extensively during testing and find it very easy to use and does not seem to interfere with my shooting. As a matter of fact, having the data right there allows me to keep my eye on the viewfinder, and makes for one less piece of kit to leave on the dock. I am using a 45 degree finder, so cannot speak if that holds true for a straight eyepiece.
It is bulky and heavy above water but while diving I did not notice the added weight.
With all this said, I must disclose that I am working with Hollis (a division of Oceanic) on the Prism ll project. But, if I did not like the mask, I would have said so, nor did they give me one, or offer me a discount on one. Bugger!