They work. I feel the transparency is more of a gimmick, or a desire to garner the same attention from buyers as the Steyr waffle mags for the AUG.
To clearly see the rounds in the mag, you need to turn the gun at least 30 degrees off target, or remove the mag. And you can't see any of them in the mag below 15 rounds without dropping the mag, because they are all in the magwell now.
The windowed Pmag does it better with a stripe on the mag spring that lines up with a round counter molded into the side.