Do you mean the knob on each radiator that lets you turn it up or down in relation to all the other radiators in the house?
If so, yes fit them.
They WILL save you money unless you for some reason replace the radiators before the payback time - and why on earth would you do that?
They mean that you don't have to pay for heating rooms that no one is using (Talkinpeace why do you heat the spare room at all when it's not in use?).
They can help compensate for rooms in your house needing uneven heating due to different insulation, being above the living room/not being above it, someone being out this evening who would normally be in their room, you only having it on for half an hour to get the washing dry, etc. Circumstances.
Best thing since sliced bread and thermal fleece-lined tights, IMHO.
Of course you only make these savings if you understand the controls AND USE THEM. And teach everybody else in the house to use them. But fuel is not going to be get any less expensive, you could save more than current estimate if fuel prices keep rising like they have been.
Note that with some types of boilers, even in the summer when it's baking you need to keep at least one radiator in the house set to less than the coldest setting. This is so that "instant hot water" boilers can release the small amount of excess hot water they produce in between you turning off a hot water tap and them stopping heating the water for it. You will not feel the radiator get warm but you can bugger up your boiler by not doing this. Suggest masking tape over one of the TRVs until autumn = easy fix.