i am rather happy with S2 IQ at base iso, except the CA problems discussed in a different thread here. beyond base iso i am not to happy...320iso is mostly ok, 640iso is hardly usable for critical work. i attach an frame taken with the 180mm lens at 640iso (full image) and a 100% crop. you can see a lot of colour noise which comes in ugly speckles. well, the face migt be a bit underecposed but nevertheless.....no manipulations were done after converting to jpg in LR3 with NR and USM turned off. this is much worse than the M9 in my view (even when doenreszed to 18mpx).
i'd be interested to hear other users' experience on these issues.