I spent a month and a bit at Koh Pos Guesthouse, and it has both ups and downs. It is situated perfectly, just 5 minutes away from the beach and the bars, and is relatively clean.
On the downside, my room had no curtains: which meant sleeping in was impossible. The wifi was inconsistant.
It has no restaurant, so no breakfast available, and no one at reception speaks except for one polite lady.