In my opinion the beaches of Sihanoukville are some of the best in the world,. They are a lot less well known than those of the neighbouring countries (especially Thailand) and so there are fewer people on them!
I really loved the islands of Koh Rong and Koh Rong Samloen: they are the busiest but they are still little corners of paradise where I felt a bit like Robinson Crusoe. If you want something that is even more unspoiled then try the islands of Koh Chaluh or the Bamboo Island.
If you like sporty holidays, then I think Sihanoukville is the ideal place! You can quad bike, buggy, dive or snorkel.
Sihanoukville is divided into two parts: Serendipity beach, in the centre, is used by back packers, whereas Otres beach, about 5 kilometres from the centre, is much calmer and well preserved.
I wasn't all that interested in visiting Sihanoukville, having been put off by its reputation, but you have to come here to get to Koh Rong so I had no choice.
I didn't really visit Sihanoukville city centre: almost as soon as I arrived I took a tuk-tuk to Otres Beach, which is the furthest one away. It is located at a distance of around seven kilometres from the centre. Upon arriving there you discover an immense expanse of white sand along the edge of which stand a succession of bungalows, restaurants, bars and massage parlours. It's touristy but pleasant enough. You can find everything you need there; you hardly even have to move. The sea is beautiful; it's perfect for relaxing and taking it easy, and there is good food and drink available. Some of the bars are noisy and don't shut till late, which is something to consider when looking for a place to stay!
The next day it was time to head back to the port to catch a boat to the island of Koh Rong. Here once again, before you reach the landing stage, there is a whole series of bars, restaurants, rental agencies and diving clubs in dizzying succession! In short, there's little chance of getting bored here from what I've seen