I find the grips great when the weather is a bit cool, and they are very handy as you can just turn them on / off as desired, setting one seems OK to me most of the time.
By the time I need setting two it is pretty cold and although they keep the inside of my hands warm the outside and finger tips still get cold, especially when commuting where I keep having to use the cold levers.
Just fitted muffs and they are brilliant, anything below 5-6c was a bit cold with my gloves (lined, but not great thick things as I like to feel the controls) and I only put the grips on L1 when getting on the bike leaving work (gloves are cold from being in top box all day) once warmed up I can turn them off completely in temperatures where I would still be suffering with grips on L2.
Grips are perfect for Spring / Autumn and the tops of high Alpine passes, but the depths of winter needs muffs IMO.