It works! Classiseal sticks like sh*t to a blanket and stops water getting down the back and shower trays and causing an issue. Used it for years now and never had a problem. It is pricey for what it is though. Mind you its much cheaper than having to take a shower out due to it leaking through a ceiling!
My tiler recommended this product as the top product to seal my bath/shower. We are very happy with the result, he says it will stay sealed for years and years.