To Serotoninsid: the cheapest ones are not neccessary the best-neither are the expensive ones. Get a reputeable brand, keep the receipt and look out for the guarantee time. Some manufacturers give more then the standard two years. The problem with a defect TRV lies not in the loss of mney/replacement costs(they're cheap enough) but in the working hours the plumber would charge you for the job.
So to keep these (maybe) re-occuring costs low go for a standard product.
The standard TRV has a standard valve. Only the temperature sensor and the expansion cartridge differ from other products - as well as price and design. So once a standard valve has been fitted to the radiator and the sensor breaks down the valve can be left on the radiator. Only the handle containing the sensor and the expansion cartridge would have to be exchanged. No mess with dirty water, the system stays closed during that operation. So you could change the design of the TRV all the time whenever you wish, upgrading from a manual to a fully remote controlled one with a timer if you wish.
To Pennypincher: Most EU countries have a binding paragraph in their building regulations demanding TRVs on all radiators. It would be illegal not to have them installed. No planning permission, similar to the rules for the insulated window glazing. It really makes financial sense to install TRVs, these laws are not made to employ the plumbers.To whom it wouldn't matter if he installs a TRV or a simple tap valve, the job is the same.