Ok , here's the deal: Prior to my visit to the Big Island I did A LOT of research wether to drive up Mauna Kea in a 4x4 or a regular rental. For all of you that are asking the same question, let me tell you this: We did it in a two-wheel drive Nissan Altima and we where perfectly fine! There is an unpaved road for less than 5 miles between the visitor center and the summit.
Although the weather in the valley was very rainy the road itself was dry. Once you're above the clouds there is was no problem at all. Maybe we where just lucky but I don't see the point in renting a 4x4 for double the price. However I cannot advise to do as we did. The drive up a regular car worked out for us - it might not for you.
Whichever way of acending you choose, Mauna Kea is worth it. Awesome views and maybe the most beautiful sunset I've ever seen.
At last, two more things:
1. bring warm clothes and GLOVES!!!
2. if you can choose, don't go during full moon. The moon is too bright and you can't see as many stars.
Anyway, have a great trip!

Although we had a rental car, we decided to visit the summit with Mauna Kea Summit Adventures. If you want the full experience, I would recommend this tour. We were driven to the visitor center aboard their bus and there we had a yummy meal of hearty veggie lasagne and a brownie. We were also provided parkas and mittens for the trip. Our tour guide gave information all along the way about the Big Island and about Mauna Kea. At the summit we saw the large telescopes (although you cannot go inside) it was a surreal experience. You are literally above the clouds and we saw a beautiful sunset. Don't forget your camera! As we arrived at the summit, some of the scientists were arriving and we got to see one of the telescopes being opened and aimed. Shortly after sunset we were taken to a site where there were two large telescopes set up and we were able to see constellations and planets and were taught about the night sky. We were also treated to coffee/cocoa and a snack while viewing the stars. What a fun night it was and I recommend this for people of all ages.

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.

Awesome views, too many clouds to see the stars so it is a crap shoot, and no entry into observatories. That was our experience. Regardless of the negative things this is definitely something to experience.