It's pretty great. My commute is 25 miles one way, four lanes of highway all the way to the office. I drive between 75-80 back and forth with no issue, as does everyone else. I have never tested the limits above that and I don't really need to. I am rarely in a rush, but it's nice just being able to get where you're going without wasting time. On the other hand, they do not play with DWI/DUI in Arizona. If they even think they smell alcohol, you're done. The highways are over-engineered for the population. Everywhere is 4-lanes, each direction. There are two loops around the town, two highways crossing through the middle and much of the areas on the loops aren't even populated yet, but have already received new blacktop. Even during rush hour, it's rare for traffic to slow below 40 mph, much less stop except for one choke point. It's the exact opposite of driving in Baton Rouge.
I took I-40 maybe from one end of the state through the other. Only stopped once at some town with a Target and Cracker Barrell maybe 70 miles west of Flagstaff. No cops anywhere.