I went to Siem Reap once ... then had to go again. There's plenty of temples that very few tourists go to if you want a more quiet experience. I would really recommend hiring your own driver on a day rate - it works out cheaper and you always know you have a person waiting for you. Much less stressful than renegotiating destinations and prices at every stop. A good driver can also help with local experience too. I had a mixed bag using tour guides. Some very great - most knew less than I did about the temples - at least that they could communicate in English.
I so want to go back again.