Siem Reap is a great place to go but try as much as possible not to just visit the temples, they get very stale very fast, try going to the tonle sap, the villages if possible etc...
Also my advice to photographers is BRING A BACKPACK!! It's a real pain (and danger) to climb up and down the steep steps of the temples lugging a tripod, camera, camera bag. Trust me it's worth the investment.
Also, people there are very friendly, even the people who keep pestering you to buy 5 bracelets for $1, if you just smile and decline nicely then start to chat with them you will find out that they are just making ends meet, most of them are also kids so you can just chat with them ask them their name, etc etc.. Most of them are more than happy to just talk to you.
Also, I would recommend taking the tuk-tuk over any other forms of transport (car and motorcycle) as you can get some great photo opportunities from the tuk-tuk over any of the other two... Try to get a good tuk-tuk driver, ours were very friendly, we even spent New Years Eve at one of their houses in his village and had snake (yes snake) for dinner...
However, cambodia is a VERY dusty place so digital shooters becareful when you change lens...
Safety wise I never once felt unsafe while I was there and in Phnom Penh so don't worry about taking out all the big guns and lens, coz there are a lot of others there who are doing the same...
Yups, to those going there have a good trip, it's really a lovely place to visit.
If you want to preview the things you might see there (and in Phnom Penh) you can visit my gallery at
www.studiov.com.sg/Cambodia