I'm considering the same, the safety aspect as highlighted by David Dublin is a concern but I am now thinking of going with a semi-sunken bath and possibly adding a rail around it.
A sunken bath I think would be kind of awkward to get into, unless you have steps down to it, have to lower yourself down and lift yourself out of it, like a pool, whereas a raised bath with teh mosaic around it style, seems more practicle.
I have never had a sunken bath, but I love the look of a bath that sunken in a raised platform. You usually see these next to a window or in a corner and have a few steps up to them and then the bath is sunken in the floor. I think it makes an amazing feature. And you would not have to worry about the safety feature.
I do think it would be much harder for an adult to give small children a bath in a sunken tub, would be much harder on the back. If this is to be the only bath in the house I think it might be a bad idea, but if it is say in an en-suite I think it could be a great feature.