Most people who have underfloor heating love it: having warm feet and a cool head is very much more comfortable than the other way round. It is claimed (admittedly by UFH manufacturers!) to be up to 20% more economical than heating using radiators; this is not an unreasonable claim as a well-draughtproofed room with UFH can be comfortable when the air temperature is around 18C, compared to 20C or more for radiator heating. The absence of radiators leaves wall space free for whatever arrangement of furniture suits you. And a warm floor is comfortable to sit on, whether for children or casual adults.
Not all rooms can be fully heated by UFH. The amount of heat that can be emitted from the floor area (without it being too hot for comfort or safety) is limited, so UFH works best for rooms with a relatively large floor area compared to the area of their external walls, windows and doors, and where external surfaces are well-insulated. Smaller rooms with poorly-insulated external walls, windows and doors will require radiators (or other heat sources) to achieve satisfactory temperatures. Even so having warm floors can make rooms — especially bath or shower rooms — more comfortable.
By the way I am generally referring here to 'wet' UFH, using pipes carrying hot water from the boiler, rather than electric UFH. Electric systems are suitable for small bathrooms where only a small area is to be heated and the aim is to provide a warm surface underfoot rather than attempting to heat the whole room. For larger spaces the expense of using electricity for heating makes it unattractive. Even in an all-electric property wet UFH used with a thermal store heated by off-peak electricity would provide a better system than either peak-rate electric UFH or storage heaters.
Installing UFH is a matter of opportunity: if a floor is being laid or re-laid for whatever reason it is obviously vastly easier and cheaper to install the necessary pipework than if it is retro-fitted. There are, however, a few fairly low-profile systems which can be used over an existing floor; these will will slightly reduce the effective height of the room (which may be an issue for rooms with low cielings) and will require doors to be cut to clear the raised floor surface.
When a solid (concrete) floor is being laid it is so little extra work to install UFH that it is almost folly not to do so.
The insulation under the concrete slab should ideally be thicker than for an unheated floor, and pipe obviously must be laid before the screed is poured. There are various systems for supporting the pipework in its required layout: using barbed 'U'-shaped plastic clips and matching rails (as in the installations below), clipped into dimpled plastic sheets (like oversized 'Lego' baseboards!) or other proprietary systems, or fastened to steel grid 'rebar'.
The pipework layout must be properly designed to give a good heat distribution, in a pattern that physically accomodates the pipework without crossing-over and without excessive length to any segment of pipe (otherwise heat cannot be pumped round it at the required rate). And physically handling and laying 50 or 100 metre coils of twisty pipe can be a challenge!
Pipe laid in a spiral arrangement for a solid floor. Note the bare areas to the right and back of the floor: this is a kitchen and will have units over the unheated areas.
Pipe laid in a serpentine arrangement for a solid floor. (This was not one of my installations: I found this photograph elsewhere.)
It is not nearly as straightforward to apply UFH to a wooden floor on joists. Schemes include laying pipe in shallow troughs just below the finished surface, filled with a dry sand & cement mixture, or using aluminium sheet 'spreader plates' which have a groove pressed into them into which pipe can be clipped.
As with solid floor installations the pipe arrangement needs proper planning.
Pipe laid in shallow troughs in a suspended floor.
In a system with a mixture of UFH and conventional radiators the heating water (from the boiler) has to circulate at temperatures of up to 80C to achieve sufficient heat output from the radiators. Water at this temperature circulating around a typical UFH system would make the floor far too hot for comfort, and could even damage the concrete screed, so some form of temperature-reducing system is used to circulate water round the UFH at around 40-50C. Where there is more than one pipe loop they must be arranged each to carry the right amount of heat. The assembly of plumbing components used to achieve these functions is usually known as a 'manifold'. Various commercial manifolds are available, or a custom unit can be made up.
The UFH-heated floor(s) can also be arranged to be heated in sync with radiator-heated rooms or can have separate time and/or temperature control.
A custom-made manifold driving several UFH pipe loops at reduced temperature using a thermostatic mixing valve and pump.
There is more on Underfloor Heating, with links to manufacturers' websites and design guides here