I've made more than 30 trips through the years to Death Valley but only a couple of times in May. However, I've traveled throughout the southwest during summer and have enjoyed temps in the 120s in the middle of the night.
It does get hot during midday to late afternoon, ut that's when the light isn't the best anyway except late in the day. Work the light at the edge of the day and spend the rest of the time napping in the shade and swilling beer, water, & gatoraide.
Overall, the desert light in the summer can be beautiful but quite deadly if you are not prepared and in shape and making sure both you and your vehicle are prepared for the heat.