We really enjoyed our Golden Circle tour. I agree with Hlitner that self driving is easy in Iceland, but we learned SO MUCH from our tour guide, that we found the price more than worth it. Having a driver (it was a minivan tour, just a few bucks more expensive than a full sized bus) take us around on our first day in Iceland and hearing him talk about the country as a whole and the history involved with the Golden Circle really turned it from a great day to a memorable one, that has led to us returning to Iceland again and again.
We self drive now, but we always take at least one tour to learn more and to be able to ask questions and meet more people.