We stayed at Grimsstadir Guesthouse for two nights in late August 2012 while visiting Jökulsárgljúfur National Park. The owners, Sigga and Bragi, were very gracious, welcoming and helpful in planning our day long hike in the park. The meals Sigga prepared were delicious, the best on our trip.
The location feels like it's in the middle of nowhere but that's how it is in this part of Iceland. We had a hard time finding accomodation anywhere near the park but this fit the bill perfectly (only 15-20 miles to Dettifoss, which is a must-see).
We here that a Chinese company is planning to build a huge resort nearby so check out the status of that if that concerns you.