When searching for car rental reviews, looking into Go Car Rental Iceland (www.gocarrental.is), I kept accidentally turning up reviews for the similarly named Go Iceland Car Rental (most of which were bad). Didn't have much luck finding online reviews on Go Car Rental Iceland, but they came highly recommended from another business I have reason to trust, so i took the risk. Now that I've booked a car with them, I wanted to post about it, in case other people are in my same position searching for info.
First, the manager, Johnny, is fantastic. One of the best customer service standards I've seen in any business big or small. I tend to ask a great many questions (in multiple waves spread out over days), and never once was Johnny anything but happy to help out with detailed responses. He also sent me a great deal of additional info to help plan my trip to Iceland, and provided a great many useful tips.
Second, the fleet of vehicles seems to be well maintained and of fairly recent years (nothing like what some people have told of horror stories of some budget rental places, where the cars are ancient). This is especially important given the prices (which are significantly lower than the big chains, by a wide margin) - I doubt a better value could be found. I went with a Ford Escape, and the prices for the same car at the big chains ranged between a 50-100% increase in price, which of course meant a huge savings (which I used to invest in better sleep arrangements).
Third, they offer a great many useful extras for reasonable rental prices (sleeping bags, gas for cooking, tents, etc). These are things the big chains didn't seem to offer when I looked. For gps, they are significantly cheaper than the big chains when prices out over a week (it offers a flat rate of 50 euros).
All in all, I have had a really good experience, and would encourage others to try them out!