Well Forte Top End Treatment which goes in with teh Oil is good, as is their engine flush. Have also used Forte Fuel System Cleaner and Gas Treatment which are both good. Most of the petrol additives clean teh system out and help injectors. Even ones from STP, Slick 50 and Wynns have made a noticable improvement on my motor when you have used the tank of fuel you have added it too.
Additives do seem to give a more responsive and smoother feeling from the engine.
If you think about it, you clean the outside of the car but never thnk about internals getting dirty etc and that is what most of the additives do, remove build up and un clog.
Beware of over use though as some like Forte or Wynns Pro range are only supposed to be used by mechanics. You can now get on ebay but as they are more effective and perhaps use more concentrated chemicals over use could be a bad thing. Just use fuel additives every 5-6k and oil additives everytime you change.