Well, I've done it. I went to Amanjiwo for a couple of days this weekend, and this place is fantastic. The service levels are unbelievable, and the place is beautiful. The rooms aren't as opulent as those of the Amans in Bali, but the experience is better. Also, the Borobodur temple is more impressive than what most of Angkor has to offer (except Angkor Wat, I think), and the restoration work is better.
The hotel has 35 rooms, of which only 4 were occupied, which makes for a whopping 10% occupancy rate. I don't know what the economics of Aman are like, but they better be pretty good to support such occupancy rates for any extended period of time. Their policy of not discounting is an interesting one, and I wonder how well thought out it is. Even the Peninsula, MO, FS, and RC will have offers when things are slow (like now).
As for safety, I did not feel unsafe at any point in my travels, and the Javanese we encountered (not just at the hotel) were extremely friendly, and happy to see anybody come and visit. My work takes me to some pretty dodgy places, and I am pretty sensitive to the general environment. I did not sense any issues in Java, except for the usual chaos that comes from being in a poor country.