I just back from the Big Island a couple weeks ago.

We rented a 4WD Jeep Wranger and drove to the summit. If you go on a Sat or Sun (which we did) there is a volunteer who leads a group of private vehicles up to the summit and you get a guided tour including a visit inside the largest telescope facility. I recommend this if your schedule permits. The saddle road itself is no worse than many roads in the mainland US.

The road is half paved and half gravel once you leave the visitor center. Our 4WD Jeep made the drive without any problems, and I even saw a Chevy Malibu driving up successfully. It's a steep gravel road and you'll burn a ton of fuel at that altitude, but it's not a technical road at all. I wouldn't necessarily recommend less than a 4WD but it's possible. If you have simple common sense, respect for the conditions, and aren't an overconfident moron the road is perfectly safe without issue. Our drive down into the Waipio Valley was probably five times as technical and dangerous. I personally always prefer to take my own vehicle when possible, because I don't have to adhere to anyone's schedule other than my own.

The only caution I will advise is regarding the altitude. 14,000 feet is very high, and it's impossible to properly acclimatise. We are very active 20-somethings and still felt strong effects from the altitude. Most visitors staying at sea level on the coast will essentially be going from zero to 14,000 feet in under three hours, and you WILL feel the altitude due to lack of acclimatization. We felt dizzy and experienced headaches and loss of fine motor function, but this is partly due to the fact we spent four hours at the summit. One lady in our group fainted after half an hour at the summit. If you're elderly or have any health problems you might want to reconsider.

Overall it was a great afternoon trip, and the sunset was amazing. If you have a proper vehicle and feel you can handle the altitude I recommend going!