I owned a BMW while the IS300h is my wife's car. Participated in a customer clinic where I gave feedback that compared to my then IS250 that I would buy the next model IS if it were a hybrid, had better ride, less road noise, folding rear seat backs and more rear leg room. Well 3 years after the clinic they delivered so we bought. The ownership experience has been generally good, except for its powertrain.
I agree that the power delivery is awful for keen driving and no different to a Prius really, but fine for sedate and normal traffic. Actually I find the latest Prius's are more responsive. So a fun winding road is best enjoyed in another car. Shame Lexus did not make the hybrid system more reactive to the keen driver. You can have some control in manual mode so long as you don’t hit the kick down button but the slippage feel still exists. A real disappointment considering the real ability in handling it has, partly due also to its 50/50 weight distribution.
I recently drove the new 320d G30 BMW. If anyone wanted a quieter car that does the benchmark acceleration around 6 seconds (vs 8.5 secs and it does feel that much more responsive) and has a fuel range that surpasses the Lexus this is it. It joins the autobahn and accelerates up to speed with the torque feeling of a V8 where the Lexus would be screaming in revs. I never really heard much sound from the diesel at all. Of course, can’t be sure of its durability as yet. But my wife's IS300h did require a replacement hybrid battery at 90,000kms.
I would be keen to see if Lexus will finally modernise the hybrid system. In the meantime, try the Camry hybrid, which at least is quite a lot more peppy than the IS.