The vitotronic 200 plugs in to the boiler front and allows you to change the room temperature programme heating times, allows a temporary boos, holiday program and an economy button just like an on-off controllers, in short you have all the control you need or want.
The good thing about weather compensation is that it holds the house at stable temperature when it is programmed to do this. With the viessmann you have two temperatures, the set back should be set at about 6c below the room temperature. This way the boiler merely tops up the thermal fly wheel of the house with warm, not hot radiators. ALso for this reason the radiators may be warm when you dont expect them to be so.. but that would be because the temperature has dropped outside and by putting heat at a similar rate as to the heat loss gas can be burnt at high efficiencies..
apart from comfort and automation, the other benefits probably include a longer boiler life as the boiler ticks over at low outputs except in hot water mode, and low maintenance as the condensation self cleans the heat exchanger.
If a boiler turned off will save the most gas and not wear out then one that ticks over at low output must last longer than one that is lurching from extremes, as well as saving energy. Of course energy savings will depend on your lifestyle....but you can also get the boiler to behave like a simple on-off boiler.
The UK industry has a funny attitude to this technology, but that is certainly not your heating engineers fault. Although almost all boilers have this technology it is ignored for reasons that are not entirely clear. BG and worcester bosch say its too complicated for the public, installers don't get the support they need because boiler manufacturers just want to sell boilers. Viessmann (the 100) and Bosch(the junior) both produce simple boilers just for the UK because we dont have civll servants who appreciate the role of technology in comfort and fuel efficiency..
As for savings... alot will depend on how you use it...if you are replacing a condensing boiler this is the way definitely to go...its also silly not to install it with a new boiler, after all it is the way the boiler is designed to be installed as most of the software is on all boards, the only exception being worcester bosch junior series, alpha and a few others..
if your heating engineer wants to talk through the finer points of the installation of the boiler he can do so in the combustion chamber or the argi websites....