Very open field.
No viewing from around the terminal when you are not a passenger.
I cycled to Ankor What every day from Siem Reap and detoured past the airport one morning.
Easy to find a spot along the airport fence. Check timetable before you go, to make sure there are movements during your visit.
Saw several local An24, Air Asia 737, Vietnam A320 and Bangkok Aw ATR.
Well, visited the place in Octobre 2008 and the situation is still the same as described by Streep. No viewing from around the terminal but you should be able to find a plce near the fence. At the moment Vietnam Airlines is the biggest operator at the field following the belly up of the Cambodian airliners.
rgds,
Chris
PS. I'm still looking for the reg of a PMT Air MD80 stored left of the terminal. So, if you succeed in reading it off (if it's still there when you visit), please let me know.