Interesting, I would be curious to know which reviews you refer to. The only poor review I found before I bought one was Clarkson criticising the gearbox for being too eager to change up/down when driving in auto. I have found with mine that, particularly in sport mode, the box seems keener to change down when deriving leisurely across country or in town than I might choose, which is why I reserve sport mode for when I need it. However I personally feel that criticising a performance car for attempting to match gear to speed in a performance oriented way is like criticising the engine for having too much power because most of the time you don't use it.
Incidentally I had go in the latest C63 on a track recently and I was struck by how much more drivable the IS-F is. The traction control in particular in the merc is really intrusive and knocks the engine right back so you have to get the turbos spooled up again to get any get up and go. It's amazing how much sweeter to drive the IS-F is.