I'm starting to use the A900 more often these days, and I'm totally impressed with image quality and film-like colors it produces. I'm also starting to realize that one has to consider it as an ISO 100-400 camera, otherwise disappointemnt will be inevitable. I used it the other day at ISO 400 as the light was getting low and I had no tripod. There is so much noise in the darker areas of the image. At 1600-3200, the noise is impossible to remove even with dedicated noise reduction software without killing all fine detail. One characteristic which seems odd to me is some chroma noise which looks like multicolored large dots, really large like 5-10 pixel diameter one dot can cover some important detail in a shot like an eye or a nose in a group shot.
I would certainly use this camera at higher ISO for personal use, but for paid jobs I would not go above ISO 400. I have to say though that at ISO 100-200, the colors this camera gives me is so satisfyingly rich and deep that this alone makes me forgive the abysmal high-ISO. I have always done paid jobs on a tripod and lowest possible ISO anyway.