Disclaimer: we didn't make it all the way to Mack's. We've got a long, low clearance ford transit pulling a Honda CR-V and had some bad luck lately with long, windy dirt roads. So we pulled off 73 well before Mack's and camped at one of the established sites closer to the highway.
Found a number of decent trails and followed them to the tops of mountains for some great views. Campsite is close enough for a day trip to Vegas or Hoover dam, lake Mead area. Temperatures are much cooler than in the city.
Not a lot of wildlife, some lizards and birds.
Very dry, and something in the air got my allergies going pretty good. An alarming amount of garbage. We cleaned out multiple bins of broken glass, cans, stone (a whole countertop's worth), automotive scraps, discarded camping gear, you name it, from our site over just a few days, and the whole area looked like this.