[Porsche-O-Phile]

My understanding is that PEX tubing is very efficient at insulating the fluid flowing inside (I've seen it used for long runs from heat pumps to heating units and stuff like that). Based on that I don't know if it would work well for actually heating something itself - you want to transfer the heat FROM the fluid through the tubing to the slab to the space. I'm sure there are ways to do it but I've not designed or dealt with a system of that specific type.

I've seen some heated driveways that used metal pipe and they were expensive to run and eventually started failing at the penetration points (but they did last quite a few years).

I'm sure there are ways to do it - as with most things it's all in the detailing and that's just a question of research and finding the correct detail for a given situation.

If you had a well-insulated living space you might be right about it being cheaper to run once everything is up to temperature. It's very expensive (and inefficient) to heat air to blow around like with forced-air systems... I'd have to see some actual comparative data though. Extra water heaters are very expensive to run, but they might be cheaper than a furnace...