I was in Guernsey with my girlfriend for the marathon in late August and stayed at the Grisnoir Guesthouse for five nights. The location, as you can see on the map, is up a hill a bit from the St Peter Port, which I liked because it made it easy to get to the beaches on both sides of the island. The room and facilities were very clean, the hotel has excellent breakfast, the staff was friendly and helpful, and there is a service to pick you up and drop you off at the airport, which I found exceptionally nice. This guesthouse was the most reasonably priced accommodation I could find during the bank holiday weekend, and I was glad we stayed there.
One thing worth mentioning: we stayed in room 8, which is on the street and therefore a bit loud from traffic noise. Also footsteps from the room above are quite noticeable when people are walking around. So if I stayed there again, which I would if I ever went to Guernsey, I'd try to get a room on the top floor away from the street, even if it meant paying a bit more.