Actually, it's much different than, say, 90F + 90% humidity. That's really uncomfortable, will leave you dripping with sweat and will leave you lethargic after an hour or two if you're outside doing something.

125F+ is a different beast altogether. It's incredibly dry -- maybe 2% to 3% humidity. And if there's a breeze or even a light wind, it's not at all refreshing. Feels like a blowdryer. You can go out, walk around and experience it for 10 to 15 minutes at a time, no problem. It's kind of fun, in fact. But if you, say, went for a hike without any cold water, midday beneath a blazing sun, you'd be in trouble within about 30 minutes. You can actually feel the water wicking out of you with every breath. It's crazy. The other problem is that while the ambient air temperature might by 125F, the ground temperature is much higher. Those guys who rode their motorcycles past Badwater midday when it was 128F never had a chance. The sun reflects off the dry lake bed and the westerly breeze piles all the hot air against the 6000-ft. mountain range just east of Badwater. The ground temperatures there can get 200F+. In fact the highest ground temperature ever recorded was taken there at about 210F. I actually went out at night to the salt flat to take some Milky Way photos. It was 117F at midnight. I had plenty of water, but it had heated to match the air temperature. It wasn't refreshing and was like drinking hot tap water. But because there was no sun, it was actually not uncomfortable. In fact, it was much more comfortable than 90F/90% humidity.

At any rate, the key is to stay indoors with air conditioning from about 10 a.m. until sunset.