Koh Rong Sanloem Island
Koh Rong Sanloem is changing fast. Photos and descriptions from two years ago will be out of date. There are two main places to stay: Saracen's bay or Mpei bay. There are also private one hostel only beaches on lazy beach or Mad Monkey, these may require direct transfers or a lot of walking.
Saracen's bay is no longer the unspoilt paradise it is often described but it's definitely not the circus of Koh Phi Phi for example. The is no stretch of undeveloped beach along the long bay. The hostel are spaced apart and very few encroach on the beach. More of a resort with no Khmer families seeming to live there. There are hotels with infinity pools and $15+ mains. Still a few dorms and rooms below $20 but few cheap eats, it's backpacking days may be numbered.
Mbei Bay is much more rough and ready, a decent amount of construction and the main beach could do with a litter pick. Hotels all backpacker focused and cheap eats a plenty. Nice village feel and Khmer families living there.