Is a radiant floor enough to heat the room?
Well it depends on the output of the radiant floor, but in general yes they are sufficient to heat a room.

We have underfloor heating (hot water with heat spreader plates under floorboards) in our upstairs rooms and they warm the rooms quite well. The floors are warm to touch (as would be expected). The temperature of the room is more uniform the heat is rising from all parts of the floor rather than from one section of the wall. We don't feel we need additional sources of heat in the rooms.

The rooms do seem to take a little longer to get warm than other rooms with a radiator, but the wooden floors may contribute to this. If you are heating tiles in a bathroom then I'd expect that to be quicker and as quick as a radiator.

I can't give comparative costs, but our heating bills don't seem excessive when compared to other people's with similar houses.