I don't own one of these. Searching the web, I found many positive reviews, and a few seemingly spurious negative reviews. This negative review from Engadget, however, sounds pretty reasonable:
"[The MacBook] was long gone by the time we got around to hitting it with a hammer. What did our final round of abuse prove? Just how little protection the sleeve offered. Each hit resulted in a perfectly clear imprint of the hammer head in the lid of our already mangled laptop. For the occasional, accidental drop or bump the squishy exterior maybe sufficient, but we wouldn't recommend treating it like an impenetrable force field for your gear."