I don't really find much difference in the IS300h responsiveness compared to a Lexus with an auto transmission, say a GS430 - the hybrid system seems to have been designed to provide a very similar, somewhat lazy, feel.
Even my F will hesitate if I'm in normal mode and I push down the throttle, but not all the way and press the kick down button, whilst it tries to accelerate in the current high gear before it decides to drops down 3 or 4 cogs and take off.
But to route66's point, things improve - the IS300h/RC300h/GS300h has a hybrid system that is 2 generations out of date (well at least 1.5 generations).