Switching between the drive modes is seamless, with no chug or shudder when the gasoline engine comes to life. The gasoline engine is very quiet, perhaps because it's so low revving (it tops out at 4800 rpm); it doesn't need to rev very high because it's primarily recharging the battery, not powering the wheels. Even when in gasoline mode, the Volt works sort-of like a hybrid, shutting down the engine at very low speeds. Because the car in EV mode is so silent, flashing the high beams will also get you a soft bleat of the horn, for use when warning pedestrians of your presence (rather than blasting them with the regular horn).
Mileage and range
When I picked the car up, it was running on stored battery energy, and indicated 27 miles of range. I made it almost all the way home under electric power, but the battery ran out after 24 miles (the driving was mostly at moderate highway speeds, with lots of hills). Indicated range is a guessing game, as a car can't know how it's going to be driven in the miles ahead, but the Volt was able to predict far more accurately than the Nissan Leaf I had recently.
After running down the battery getting the car home, I plugged it in for an overnight recharge. (A full recharge using household current takes about 10 hours.) The outlet is behind a fuel-filler-style flap on the left front fender, and the extension cord stores in a cubby under the cargo floor. Plug it in, and a light on the top of the dash glows green to let you know charging is taking place. When the battery is topped off, the green light flashes.
The next morning with a full battery, the car showed an available 36 miles of range. In fact, it wasn't until I'd driven 41 miles (most of it around town) that the gasoline engine fired up to keep the battery pack from running down any further. The only sign that that happened, by the way, was that the instrument screen changed from showing the battery indicator counting down the EV range to a gas pump graphic showing 295 miles of estimated fuel range (from the small, 8-gallon tank).