Angkor Eye is a modern Ferris wheel on the northern side of Siem Reap, useful less as a thrill ride than as a change of perspective. From the top you get a clean look over the low city, the hotel strips, and the flat green country that leads toward the Angkor temples. It is especially pleasant near sunset, when the heat drops and the city lights begin to come on.
It will not compete with Angkor Wat, and it should not be planned as a substitute for a temple-viewing spot. Treat it as an easy evening add-on, particularly with children or on a day when you want something light after hours of stone, dust, and archaeological detail.