My wife and I stayed at Hang Guesthouse for two nights in February 2010. It is a cute, family-run place on the river just south of the promenade. Kampot is a tiny town, and you can walk from one end to the other in 20 minutes. Hang offers free bicycles, and it's possible to ride from the guesthouse to the center of town in about 5 minutes. There are few street lights, so it's helpful to have a flashlight if riding in the evening.
We had a double room with A/C and hot water for $22 (breakfast was not included). The room was spacious, with a comfortable bed and a mosquito net. Water pressure was a bit light, but hot water was abundant. We made use of their laundry service, which was cheap and quick. They also booked a reasonably-priced day trip for us to Bokor Hill Station ($20 included transport, guide, lunch, water, park admission, etc.), and a taxi to Phnom Penh ($35). We ate breakfast at the guesthouse once -- it was a bit more expensive than in town, but still quite reasonable by western standards. The veggie omelette was delicious. Staff were genuinely very friendly and helpful. The wifi was down while we were there so they offered us the use of their personal computer anytime we needed access, and he also called hotels in Phnom Penh for us.
We had a very comfortable stay and I would recommend this guesthouse to anyone visiting Kampot.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.