Stephen,
No personal experiences myself, but studies shows that zeolite can take up water up to 70% of its own weight. Seramis does a better job and can suck up more than its own weight with water. Dry Seramis weighs 380 grams per liter, but completely soaked it weighs 800 gram per liter.
Zeolite is mostly used in agriculture as a slow releaser of fertilizer which it sucks up greatly. In combination with its water retention abilities therefore beneficial in dry agricultural areas. I don't know if Seramis has the same benefits when it comes to taking up fertilizer.
Our Wageningen University did a test with potatoes growing in soil enriched with zeolite in which fertilizer was added. The crop was 11% higher than the crop growing in a non zeolite enriched soil. Although significant the University said that it was only one field test, so several more needed to be done to conclude a pattern.
Best,