I have an '06 6s with the Aisin-Warner 6-speed auto and I have to say, although it never slips, it does occasionally gear hunt, is slow to downshift and is not always the smoothest operator - particularly with the A/C on for some reason. I think most of these problems can be solved with subsequent firmware updates, but nevertheless, it's not as rock solid as I'd hoped. Especially for a vehicle with sporting aspirations that makes most of its power in the upper RPM band.
My personal opinion is that I wouldn't buy a used 03-04 Mazda6 automatic, unless you know the vehicle's history and that it was adult-owned. With the manumatic function and trouble-prone history, it's too easy to pick one up that some kid beat on and be left with what could potentially be a very expensive rebuild/replacement. JMO, flames will not be responded to.