We booked our tour with Mauna Kea Summit Adventures and are very glad that we did; the road would've been impassable without a 4WD vehicle and so we were very happy to leave the work to the experienced drivers. We were picked up at a shopping center in Kona and driven to the visitors' center where we ate dinner (provided by the tour company) and acclimated to the high altitude. We then were driven to the summit where we watched the sunset and afterwards, we were taken down to a spot not too far from the visitors' center for some stargazing. We were returned to the shopping center at around 12am.
I highly recommend taking a tour as it was really nice to just sit back and let these folks take care of everything. They provided parkas and gloves for everyone (those really came in handy at the 18F temperatures at the summit), hot chocolate/tea and brownies for folks during stargazing, and the guides were very knowledgeable about Hawaiian history and folklore and made the drive very interesting.
They also were obviously well-versed in how to safely navigate this mountain and had a system in place for when they countered difficulties. On the night we toured with them, one of the two buses broke down at the summit and wouldn't start. During our climb, the heat from the engine had melted something important (yeah...I know extremely little about auto mechanics). We did have to sit around and wait for a while until the drivers could get it restarted, but they seemed to know what to do to get everyone home safely and when the park rangers arrived to check in on us, they seemed to know the drivers on a first-name basis. Our stargazing was only a little delayed and everyone got home comfortably and safely.
Definitely bring warm clothes when visiting here; even with the parkas and gloves, we were freezing after becoming used to those 80+F temperatures in Kona. Also, if you are going to book a tour, try to book early as some of the companies offer discounts if you book 2 weeks in advance.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.