I used a similar one from Sealand. Problem with those is that they
aren't vented overboard. Even though they seal well when closed, the
"stuff" will build pressure and you get a little blast of unpleasant
air when you open it to use or flush it. Always crack the seal before
using it... if you have a bowl with contents and you open the slide
valve to empty it, you may well get a blowback. They're better than
nothing though...