I have been to Koh Samet 5 times in the last year. I have stayed at some of the very budget accommodation, which for this island is the 400-1500B price bracket, and have always been unhappy. I have then had to move to alternative accommodation, thus have stayed at many of the islands resorts in and around Sai Kaew beach. There is only one place I stayed at before mossman, and I was paying over 3000/night there, so this was not an option for me on every visit.
Mossman House is next to the 7/11 in the Koh Samet village. This means that it is before the park entry point and they say you do not need to pay the charge then to enter the national park.
The room I stayed in was 1100/night, and matched the standard of the other place where I was paying 3000+. Clean, spacious rooms with air-con, fridge and en suite bathroom with power shower. Also you can be on the beach from here in under 90 seconds. 7/11 is literally a few seconds walk so very handy indeed. Also the rooms are very quiet.
They will do your laundry, arrange trips and motorcycles, and will offer discounts if you stay for a few nights. Considering some of the overpriced, truly attrocious accommodation in the vicinity, this place is a real gem, and a pleasure to go back to after a hard day on the beach.