Definitely a ‘must do’ on the Big Island. Being from Colorado, the drive up was no big deal. 4WD isn’t necessary. I think it all comes down to weather and conditions (the University of Hawaii has a website that shows up to date road conditions. Just search for “Mauna Kea road conditions”). Conditions were great on the day we went up. The road is well maintained. We were driving a 2WD Chevy Impala with a pretty big V-6 and had no problem. There are only a couple short steep portions. The sunset is beautiful and all the observatories are impressive and other-worldly. We left the top shortly after sunset, so it was pretty light still on the drive down. We stopped at the visitor’s center where several amateur astronomers have set up big telescopes and were happy to talk about the night sky. (Don’t be a goober: make sure to turn off your headlights if you’re near the visitor’s center at night!).