I have put this in my bathroom and ensuite when they were refurbished a few years ago. It's best to use it with another heat source, we've got only a 1m-ish square mat in the ensuite (retrofit into the bedroom, so it's a pretty small room) and it doesn't heat the room. The one in the main bathroom (room's about 8x8) does a better job, but we've also got a heated towel rail. We don't have it on for long during the day (a base of 15ish most of the day, up to 25 in the morning for about 2 hours) and never noticed a hit on the electricity bill.
We put insulating board down on the old floors, the heat mat and a thin layer of leveller under the tiles. I think some sort of insulation under the mat is recommended, otherwise you're heating down as well as up - and that's a lot of concrete to be heating.