I have used the tile paint on the fibro panel walls of the shower in my IP (can't remember the correct name of it). I spent hours rubbing down with wet and dry (wet) to remove 30 years of deposits to make sure I got all the soap off - it was worth the effort. I am surprised with how well it stands up to every day use. I have done a few touch ups from the odd scratch or spot peel, and repainted the bottom 2/3 a few weeks ago after 5 years as the soap was starting to discolour the paint a little, but still shiny (I would be surprised if none of my tenants had used mild abrasive cleaners like Jiff). Being enamel the touch ups do show up against the older paint but it yellows in time so that it is not noticeable - this is the one drawback, repainting tiles white will eventually turn cream!
I know it is not recommended to be used on floor tiles but I suggested it to a mate who had bought a house which the bathroom floor had been retiled - excluding the shower. To hide the old terracotta colour shower floor tiles he painted them in a similar colour to the newer tiles - prepared well and it has lasted 5 years as well. The only peeling is a small spot directly under the soap holder where it drips onto the tiles - I had a similar issue in my bathroom where the soap holder allowed soap to drain back onto the wall and peel the paint ...must be the caustic in the soap.
I have had a problem with feathering the edge on spots that have peeled - keeps peeling back no matter how far I extend although the subsurface was well cleaned and sanded. I have spoken to White Knight to ask if there was a more suitable primer but was told no, they realise that there are limits to what it will adhere to. When I first used the paint I decided to test their specific primer against normal Taubamans 3in1 on an old tile - primed each half of the tile in each and then painted over each half with the top coat. I placed the tile out in the garden face up, no wear and tear other than leaves and sticks, over 3 years later it was still holding up and I was about to ditch it when I noticed one day when watering that it had just started to lift - on the 3in1 side. So the correct primer has the edge, although it was not far from lifting when I scratched it.
I have seen other brands of tub, tile or laminate paint used on the TV shows, I wonder how they perform ...and if any are water based to avoid yellowing?