I've just got my IS300 back after 7 days with an 09 IS250MM Auto.
I will miss the interior of the new car.
But I love the visibility, engine response, steering feel and general 'nippiness' of my IS300.
Again, I don't think the two are comparable, they just share the IS in the name.
Only thing I want to do to the 300 is to get another set of sway bars, as the new IS250 is superior for body roll/control.
If I was only allowed to choose one for life, I'd have the new IS250. But for driving pleasure, I prefer my IS300 :)
Chris