I've got it under my granite bathroom floor (tiny bathroom). It's so snug and lovely. It cost about £200 to install (it's electric, a stretchy net thing, very easy to install) and costs the same as a 60w light to run. Do it!
We renovated our bathroom in the summer. Completely tiled all the walls and floor. Decided to put underfloor heating down at the last minute and we are so glad we did. My DD said it is the best thing we have ever done to this house. Now that the weather is colder it is making such a difference. When the central heating isn't on it would be freezing in there.
To some extent it depends on the layout of your house. Our bathroom has just a single towel rail radiator but is never chilly. However, it has Amtico flooring not granite or ceramic tiles, and it only has one small exterior wall. If it was on a corner with two large exterior walls it would be a different cup of tea.