I will say, having warm floors is really nice. I think you can actually get by with a cooler setting on the thermostat because your feet stay so much warmer. Grace can run around in bare feet in the middle of an arctic blast outside and it is especially nice to get out of the shower and step on a nice warm floor. The colder it is outside the warmer the floors.
The main drawback is it's slow response time. When you have big temperature swings, the floor reacts too slowly, so just about the time the sun is coming up and the air outside is warming and the suns rays are warming the air inside the house, the floors are really starting to get warm, which is the opposite of what you actually need at that point. I suppose if you lived in an area where large (more than 15-20 degrees F) swings in temperature occur on a regular basis, you could probably just use a programmed thermostat to work out the issues with the lag.
Other than the lag, it's a great, even heat.
-John