Can't tell you the cost (depend on the system) but effectiveness wise, its fricken ace!
We had the electric matting put down in out kitchen under tiles, and though I was very dubious, it work really really well. The animals loved it too, especially as our systems control pamphlet might as well have been in Thai, for all the sense it made (download the full one from their site and it all became easy and clear). So ended up using it for the first couple of week at max, though this was in January, so no-one complained - cats and dog just spent the whole time asleep on the floor.
Surprisingly though we didn't remove the only rooms radiator, there is no need for it to be on, unless it's bitterly cold (probably not even then). Heating from the floor just seems to make the rooms a much nicer places, no hot spot by the radiator and cold by the doors. Warm tiles on a mid-winter morning are great - in fact it puts the front room to shame now (wander round in the kitchen shoes less, go into the front room & you need to get the woolly socks out!
I believe you limited to the electric matting unless you want to increase the floor height, or dig out channels for the piping. Though obviously more expensive initally, it's no doubt more economical long term. IIRC you'll need to run seperate heating circuit from the boiler, for the under floor stuff, to you normal radiator system (differing pressures?).
To re-iterate, from a complete "doubting thomas", I.m now a fan of these systems.