Looking at some of the reviews, it sounds like this hardware has its limitations, namely that it does a poor job of blocking out sounds in the frequency of human voices, which is the main thing that jhandl seems to be looking for.
I use a pair of these ear muffs almost daily; they pretty much block out all sound well for me. Here are a few reviews for the product that address your concern:
"I use them while doing research in a library. It's unbroken silence despite screaming kids and adults talking into their cellphones. People no longer understand proper behavior in a library, but I don't care. I can't hear them. It's wonderful." http://www.amazon.com/review/RQBNEZ9CX3O19/
