I don't think I am saying anything different to what other people have said already. Angkor Wat and it's related temples are simply breath taking to behold. It is an experience that everyone should do.
We did the temples in one day which while it can be done, is still very gruelling. Our guide was Sowanna and he was fabulous and we cannot recommend him enough. He took his time explaining everything to us in easy to understand language. However, we knew our guide was superior to everyone else when we encountered a Canadian tourist who had collapsed during his tour. There were so many people standing around him including his tour guide and no one was doing anything for him. As soon as Sowanna saw the situation he completely took over and started massaging him to get his circulation going and he ordered one of the security guards to call an ambulance. He was very authoritative and competent and everyone there was grateful that he was around.
So I think it is important to have a good guide, but I would also confidently recommend Sowanna because you will be in good hands in the event of an emergency. His contact details are: [--] and his website is http://www.cambodianguide.com