Read David Gluckman's comments on driving the 2008 Lexus LS600hL.
How do you make a Lexus even quieter? Turn off the engine. I pulled into my neighborhood one night this weekend and noticed that the hybrid system had gone electric-only. With the windows down, all I heard was the tires and some electric motor whir. The dead quiet was eerie.
I played with EV mode (activated by a button on the dash) and, like Phil, I noticed it's tough to keep the car from switching to normal mode. EV mode will only work below 25 mph and as long as you're not in a hurry to accelerate. If you're being gentle and it anticipates the need to use the V-8, it will beep and display a message saying so to allow you to back off the gas, er, accelerator pedal. I found it better to let the car decide when to shut the engine off (at stoplights, when cruising at constant speed, during deceleration) instead of modifying my driving style.
Compared to our recently departed Four Seasons LS460L, the 600hL is more of everything, almost. It has a larger V-8 that produces a little extra power, all-wheel drive instead of the 460's rear-wheel, more standard features, and a $115k price tag. One place the hybrid loses to the normal car is in the trunk where a lot of space is eaten up by batteries and other hybridness.
To me, the LS600hL is a big gadget. It does some amazing things like juggle the engine and electric motor nearly seamlessly; park itself; brake for you to help avoid accidents; and keep a (very) safe distance while cruising on the highway. I applaud Lexus for developing the technology that makes it all happen, but while I like electronic toys, I don't want to drive one.