Sadly the honest answer from my experience is that the heat isn't good. Here in arizona I experience a thermal cut back durring the hottest of days. I've done my best to counter it, but in the end the car never likes it much. My car's red, so not quite as bad as dark blue, but still absorbes the heat rapidly enough. What I've done is to get a car cover and custom fitted windshield cover that I use all summer while the car is parked. Between that and the immediate use of air conditioning without auto stop until the car is sufficiently cooled off I've avoided further problems most of the time.
Really it depends on this car your thinking of buying. Personally I've ot a 100,000 mile warranty on mine partially because of this. I know of several batteries locally that have been replaced that the heat pretty much destroyed after a few years.
On the other hand though, if they go out before 80,000 you get a new pack free. You adjust your driving style and it's usually ok durring the summer. You didn't specify where you live, but I think my experience deffinitely qualifies for the extreme. But, the AC works wounderfully on the Insight, even when it's over 110 out.