Yes, I installed electric underfloor heating system under stone tiles in my kitchen a couple of years ago.
It works quite well without any other radiators in the room. Very occassionally I suplement it with an electric fan heater if we are being very sedate (watching TV). However because it's my kitchen I'd have the tempertature a bit lower than a sitting room.
Cost wise, it's about €40 - €50 a month to run, without night rate electricity.
But ...
Between the supplier and the contractor that laid it I had two big problems which could have been avoided.
First, like you I had a fairly largish area to cover & I had to buy 2 mats. The supplier told me to wire the second mat into the end of the first one. This proved wrong, both mats had to be wired separately to the thermostat. I had to dig up several tiles in my very expensive floor to get to the join in the two mats.
The supplier promised me they would compensate me with a free mat, but when I went to take them up on it recently, they offered me a pretty paltry discount. I am looking at my options to sue them.
Second problem was with the thermostat & floor sensor. The floor sensor, which is a bit of wire, came with a piece of conduit. The idea is that you can replace the wire easily. Unfortunately the contractor didn't bother to read the instructions & didn't use the conduit. When the system wouldn't work & I traced it to the sensor, I couldn't replace it. I finally got it working by disconnecting the sensor from the thermostat & running it on a room temperature sensor only.