Just returned from Beijing after staying at the Doubletree. We used a tour company, so can't give any info regarding subway. The location, if you are trying to get anywhere by car (including from the airport), is not great.
We also stayed at Conrad Hong Kong, Conrad Macao, WA Shanghai, Conrad Tokyo and Hilton Osaka on the trip and Doubletree definitely provided the poorest treatment for diamond. No upgrade, only allowed the required number of people for the executive lounge (there was six of us in three rooms with two diamonds, they wanted to charge us if the last two wanted access).
The rooms are pretty basic. In fact the hotel provided by AA when our flight was cancelled was better.
Nonetheless, it was very good value for the 10,000 points per night/per room we spent. It was clean, breakfast can be taken in the restaurant or lounge and the spread in the restaurant is extensive, if not the best quality (better than Hilton Osaka, but worse than all of the others). If I was staying by myself, without my family and I did not need to get anywhere in a hurry, I would probably stay there again, based on the excellent value for points. If it was my wife and I, I would definitely spring for something better.