From the World Heritage inscription:
Between the 4th and 13th centuries a unique culture which owed its spiritual origins to Indian Hinduism developed on the coast of contemporary Viet Nam. This is graphically illustrated by the remains of a series of impressive tower-temples located in a dramatic site that was the religious and political capital of the Champa Kingdom for most of its existence.
If you visit the smaller, more distant temples in Angkor, you’ll have an idea of what My Son is like. The architectural styles are very similar and both are of ancient Hindu origin.
My trip to My Son was rather disappointing. It is about an hour drive from Hoi An and is up in the hills. On the day I went it was raining heavily so what few photos I took had to be taken under an umbrella. It is also much smaller than I assumed it would be. Even in the rain, I was able to walk around the entire grounds in about an hour. While it is only a fraction of the size of Angkor, the visitor center and facilities are much better than anything you will see in Cambodia.
If you are in Hoi An it is worth the trip, but if you are expecting anything like the temples you can see in Angkor you will be disappointed.