Matt:
The viewfinder of the S2 is very large and bright without a doubt, but it is also very color-neutral. My experience with some of the viewfinders on other brands of MF cameras has been that they often exhibit a slight green or yellow cast.
I frequently use non-Leica lenses on the S2 with the standard focusing screen, and find that the viewfinder is more than bright enough to manual focus these lenses even when stopped down to around f/8 or so.
Check out this link for a few images and a video through the viewfinder of the S2 using the upcoming microprism focusing screen. It should give you an idea of the viewfinder’s size and brightness.