How robust is ceramic tile paint?
We have a bathroom in our house where the previous owners thought it would be a good idea to paint over the tile and grout with some kind of ceramic tile paint on the walls.  The house was a rental for some time before we bought it, and while we don't know how long the paint was in use, it seems to be very durable.  

Since we have owned the house, we have not had any issues with the paint standing up to normal use and cleaning.  We have not hit it with abrasives, but I expect it would scratch in the same ways that tile would.  I haven't tested this, though.  There is one spot on the wall where someone was able to cause some grief on the wall with either abrasives or some other kind of failed cleanup.

Because the paint isn't on the floor tile, it's hard to say how well it would stand up to foot traffic, but it does seem durable.  On the walls in the shower, it has cracked a bit when some remodeling was done in the adjacent room (gutted kitchen to studs) and I believe there was a bit of flex in the wall during that work.

If given the choice, I'd use this kind of solution as a last resort.  It doesn't look quite right and while it's durable for most instances on walls, it might not be good for floors.