What I mean by this is that ISO 200 looks like 800, 400 looks like 1600, 800 looks like 3200 etc. For jpgs. they seem to add a lot of noise compensation but all this does is attempt (somewhat unsuccessfully in my view) to "blur" the noise. I mentioned in one of my pieces that the DP2M is a "one trick pony" in that from what I see, its unbelievably good when using raw files at ISO 100, but not really much good for anything else.
That works for me, and thats how I use most of my cameras anyway, but I would advise that if people are considering this, then be in no doubt that you are looking at high ISO performance that is no better than cameras from many years ago. I don't know whether this is a consequence of the Foveon technology, but I assume if Sigma could have got it better they would have done."