Cambodia is very safe. I spend about three weeks, mainly in Siem Reap, and Phnom Penh. In Siem Reap, I recommend staying at Star Rise Guesthouse. Slightly out of the main bars/club road, but its great.
Seal, the guy who runs it is awesome. As are the drivers at the GH.
I never felt threatened once, but the roads are CRAZY, although not quite as crazy as VN.
People do look at you a bit more than in Thailand, but this is just because they aren't as used to seeing western people.
You will find the motorcyccle/tuctuc drivers are a bit pushy, but just let them know you're not interested.
Also, in Phnom Penh, we stayed at Same Same But Different GH. Don't go here. Don't get me wrong, its nice, and right on the river, but it's dead compared to the place next door.
Also, in Cambodia food takes AAAGGGEEESSS to arrive, so just be patient. It's always tasty.
Also, taking the bus into Cambodia from BKK, you will be told there are no ATM's in siem reap. Don't listen, this is a scam, you will just get ripped off at the boarder due to exchange rate differences. Wait until you are in siem reap.