If you live in VERY hot weather with lots of traffic (Houston, for instance) you may have experienced a conventional car shutting off your A/C because it's overheating and can't handle the extra load. The Prius is much more thermally efficient and produces less waste heat... even in "highway is a parking lot" conditions and Prius frequently running the engine to power the A/C, overheating is very unlikely to occur. Not only does the car burn less gas (making less heat), it more thermally efficient (Atkinson Cycle Engine).

On the flip side, some have complained the car seems to take awhile to get warm in winter, mainly those in extreme cold, like Dakotas or Canada. For most it works fine. The car will run nonstop until it's warm, both for you and the engine. The electric heaters mentioned above produce heat immediately, thou they're small, won't come on if you're drawing too much juice from the 12V systems (motors and AC don't matter), and are meant to supplement the primary heating core. The car also mixes recirc air with fresh air to provide more heat with less waste (like an airplane.) Recirc air blows on the bottom and fresh on top to prevent window fogging. A humidity sensor changes the mix as needed or turns on the AC compressor to dehumidify. If you leave the system in AUTO with AC on in winter, it'll do whatever is needed to make the cabin the temp you say, using as little power as possible (for instance, AC would only come on if it's too humid).

Probably more info than you wanted to know.. but interesting all the thought that went into keeping everyone warm and toasty. Car gets much better MPG in warm temps than cold. In temperature extremes, dead batteries are a nonissue and some Canadians report car always starts up in even the coldest weather (provided fuel lines are stabalized).