Most of the reviews here are for their camping facilities. This review is for their guesthouse. The room was basic, but fine. The full kitchen was great to have! The one bathroom (for 3 rooms) was good enough (though the shower was quite small/tough to turn around in if you are full-sized).
Be ready for an idiosyncratic owner. He renegotiated the price after we arrived (due, he said, to a misunderstanding about whether we were renting a room, or for a specific quantity of people) - we eventually agreed to the change, in order to keep the peace. He pushed HARD to get us to pay in cash - we refused (both because of our own convenience, and because I wanted a way to get my money back if something went wrong - I didn't trust the guy).
Several times he reminded us that it is his house, not really a guesthouse (that's what a guest house is, sir!...a personal house that you rent out a few rooms in)...and several times walked through the kitchen (when we were there) and turned some or all of the lights off (to save a few pennies).
Broadly, I'd categorize him as a miser.
The location is perfect. The availability of the room was key. If you prepare yourself for some idiosyncracies, then it'll meet expectations.
If we were to return to Myvatn, we'd try to stay elsewhere, but would stay here if there weren't better options.