Owen;
I got a set of those "used" with my pallet jack battery setup. They seem to work as claimed at least over the whole 4 months or so I have had that bank set up.
One "gotcha" is my tester needed an added bit of tube to reach the fluid due to the added height of the new caps compared to the originals.
You can check out the system over here:
http://pics.ww.com/v/TomW/Battery/
And the caps here:
http://pics.ww.com/v/TomW/Battery/caps.JPG.html
I got these for cheap for like $25 for 12. A bit stained / used looking but they do seem to help recombine the gases that are driven off from hard charging.
Again only about 4 months on them but I would add them again as a matter of course. The flip top is easy for checking levels and SG, too. Nearly worth it just for that.
Tom