Angkor Wat is simply brilliant. I recommend that if you are travelling from Bangkok take the train to the Cambodian border rather than the bus as when you reach Cambodia you will probably find yourself in a clapped out minibus rather than the promised air conditioned coach. Although you will probably end up in this minibus anyway for the journey through Cambodia, at least you will have got the chance to travel separately from the vast majority of tourists.
Updates
Nowadays, the best way to travel Bangkok to Siem Reap is taxi, especially if you are a family. Bangkok to Poipet costs $60/70, and Poipet to Siem Reap $35/40. The whole journey, hotel to hotel takes about 5 1/2 hours. Balance this against $600 one-way for a family of 4 on Bangkok Airways, which, hotel to hotel will take you about 4 hours. Also, you can get your one month tourist visa at the border (takes 10 mins) for 1200 Thai baht each.
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