I looked into the concept last year. I researched the SunTouch system in particular. This was for a bathroom on a concrete slab in TX. Samclem has covered the problems well.
The relatively good heat conduction of concrete, coupled with the large thermal mass of the concrete, will require a large amount of heat for a long time to do any good. From Suntouch's own charts, maybe
I could get a 10 degree F rise after quite a while of running it. 10 degree rise over ambient ain't much. And the usual use of a bathroom is counter to a long warm-up interval.
The only way it would work for me is if I insulated the slab first. No way was I going to build up some big thick sandwich and then mortar big porcelain tiles over that. Concrete by itself makes a great base for tile, though a thin crack-isolation membrane can help longevity, particularly in bigger rooms. People who have no choice but to put tile over a wood sub-base who do it right want at least a 1/720 flexing moment, and they do a lot of work to get the floor that stiff. Since I have a concrete slab, it's bending moment is extremely high, no way do I want to lower it to wood-construction levels by making a squidgey sandwich
So for my application, radiant in-floor heat just isn't worth it. So I just refurb'ed the IR Heat Lamps, they give instant toasty on the few mornings it is needed. And since it is simple and overhead, what little maintenance that may be required is easily performed... and cheap!
of the floor area is not covered by the IR lamps, so if one gets the urge to do a fan-dance on a cold morning, just stay in the spotlight on the stage