My visit to Angkor Wat was a once in a lifetime experience and worth every penny. I would recommend this to everyone, except that you must choose the right time to visit. Just don't do it when I did, in December.
One of the highlights was the Baphuon temple, recently partially restored from its totally broken down state. I scrambled up to the top gopura or tower, something that any reasonably active person can do. However, do watch out for the narrow, steep climb - the ancient Khmer people did not believe in making broad, gentle staircases!
The view from the top is breathtaking in its symmetry and geometrical perfection. You get a great view of the forest around you, and your elevation entitles you to a gentle breeze that cools you down quickly. Be sure to buy mosquito repellent, carry plenty of water, and bring a large brimmed hat to beat the heat. It's hot and humid in Khmer country.