Pretty much Sihanoukville but on an island. Lots of people really love this place and before going I read a lot about the fact that it would soon be ruined by popularity and development. I'm quite sure that it's too late. I read an article from 3 or 4 years ago that said something like "expect to have the beach to yourself". Well now it should say "expect to share the beach with 100s of hungover tourists".
I think my main issue with Koh Rong is actually the tourists. From what I saw, these aren't travellers looking to explore Cambodia, they don't care about the culture, the food, the people or the country. They want to lie on a beach and then get blind drunk at night. Rinse, repeat. Of course this is a generalisation but it applied to almost everyone I met in my 4 days there. Multiple people had never heard of Amok or some of the major towns in Cambodia. As a result, Koh Rong is catering to their needs and that doesn't include good Khmer food. Actually it didn't include good food at all. I understand that the resources on the island are limited but some of the food was actually awful.
I hear that there are quieter areas of the island that are much nicer but I only stayed in the main area where the boat arrives. However, if you're looking for an island that's beautiful and quiet, rather than going here and to a different beach, it's probably better going to the separate island of Koh Rong Samloem. I've heard so many great things about it. The accommodation is limited and expensive, definitely out of my budget as I wouldn't be sharing, so I decided to do it as a day trip. However the boat guy screwed me over so I never got to go.
Electricity for the whole island is provided by a big generator that is turned off between 2am and 8am, then again around lunch time for a couple of hours. The problem is that any fan you have in your room to keep you cool (it gets very hot, even at night) will be off during the hours when you really want to sleep.
I'm being negative because, although I had fun, there are much much better places to get drunk or to lie on a beach. Loads of people love it though, so don't let me stop you!
Accommodation
White Dragon
Basic, thick walls, cheap, good location.
Food
White Rose
Big portions of western food but nothing special. Quantity over quality.
La Mami
Very basic (but fresh and tasty) Italian food. Expensive and small portions.\
Treehouse Bungalows
Pizza from a wood oven. Pretty average but better than most on the island.
Nightlife
Apparently a different place is popular every night. Although Bongs seemed to be the main place most nights with live music and the dancing spilling out on to the beach.
Sights & Activities
Boat trip
There are loads of trips leaving every day. The most popular seems to be a half day trip ($5 - $10) to do some (rubbish) snorkelling, some fishing (we caught loads!) and then to Long Beach where you eat your fish and leave at sunset. It was a nice way to meet people too.
Massage
I had a great Khmer massage at the spa in front of Paradise Bungalows (which is a nice place to chill but probably best to avoid the food based on my experience).