Experience with Sony Alpha cameras?

Serious photographers only, please. Has anyone out there used the Sony Alpha 550 and if so, how do you find the camera? That is, do you really notice the lack of ISO100 in terms of picture quality when doing macro photography? Does the lack of a mirror lock-up affect image quality at lower shutter speeds, or does the image stabilization in the camera compensate adequately for that? Does the built in flash cause unusual camera shake and effect sharpness of the pictures? How good is the image quality at higher ISO speeds like 3200? Would it be better to go to the Alpha 850? Is there a Canon or Nikon you would recommend instead of the Sony? I want at least 14 MP, excellent picture and color quality ease of use, fast continuous shooting speed.