OK, just gave it a try.
It's usable, but cumbersome.
Sound cut didn't seem to be that much to my ear, but I measured it at about 10dB. (On the Kohlert, using a Morgan 7L, 115dB max volume, 105dB in the bag - Sax was pretty loud and painful at that volume, I wouldn't want to do it too often.) A change of 3dB means roughly a doubling or halving of volume, so the 10dB cut means that playing in the bag is less than 1/8th of the volume when playing normally. About 1/8th of the fun as well, cos it was really muffled/woolly sounding. But if you've got neighbour problems, it may just do the trick without breaking the bank.
Looks to be similar to the one at Lewington's posted above. But mine is labelled Precieux, and it cost me about 10 euros in as new condition on ebay. I think the orignal buyer got it at Aldi or Lidl as one of their specials for about 25 euros and didn't like it much....