I recently stayed for two nights at the Park Hyatt Tokyo in early January. I read all the threads on flyertalk before going so will try to post some new information.

Reservation process: I booked the cheapest room at the prevailing rate on Hyatt's website, which was 46,000 yen. I used an 6,000 point upgrade award for a park suite. I had booked the room about 2-3 months before my trip, and called hyatt customer service to book the upgrade. I didn't get a response from them for several weeks, so emailed the hotel and the next day got a reply confirming me in a park suite.

Room: As previously described on this site, it is quite a nice suite and the pictures from the website are accurate. The only disappointment was, although it was a corner room, one wall of windows had been blocked out with shutters that couldn't be opened. According to the hotel it was to prevent one from seeing into the adjacent room.
I had emailed the hotel about getting a Mt. Fuji view and did get one of these rooms. Luckily I was able to see Mt. Fuji the last day, which I understand is quite rare.

Food: I ate at both the New York Grill for their holiday buffet lunch and ordered room service. The holiday buffet was 9,000 yen and included a glass of Don Perignon. (from 1999, I think) In addition to the substantial buffet you also got one entree you could order from the menu. Although the food was good, I was not blown away by the quality.
The room service menu items ranged from 2000 to 8000 yen. I had ordered a quite good avocado salad, a very average udon dish, and my friend got a small pasta dish. It was nice to take advantage of the suite and eat dinner in the room.
The basement of the building the hotel is in has about 10 different types of Japanese restaurants, with average prices of 1,500 to 2,000. The last day I ate at one of those restaurants and felt the Japanese food was better than the Japanese food through room service.
This hotel was my first stop in Japan so I was more in the mood for trying Japanese food. I think the New York Grill and room service menu would be more welcome at the end or middle of a Japan trip when one might want a change from Japanese food.

Location: I would have to agree with some of the previous posters that the location is inconvenient for tourism. It is about a 15 minute walk to Shinjuku station. It is located in west Shinjuku which is mostly office buildings. The only major sightseeing stop in west Shinjuku is the Tokyo Metropolitian Government building, but you get the same views from the hotel so don't need to go there. The Shinjuku nightlife is in east Shinjuku which feels like a long walk from the hotel.

Visiting Tsukiji market from hotel: On my previous Japan trips I had never woken up early to see it, but decided to do it this time. I took the taxi from the hotel leaving about 4:30 am. It cost ~3,000 yen. I took the subway back; the subways don't start running until 5 am. I would recommend taking the taxi there as most of the action in going on at 5 am. Very interesting and worth the early wakeup.

Overall impressions: While I enjoyed the hotel and am glad I stayed, I will probably stay elsewhere next time I'm in Tokyo. My hotel preference is to be within an easy walking distance of the sites and metro, so I'll probably try the Grand Hyatt in Roppongi next time. Also, the room rate is quite high so I'd consider a cheaper hotel and use the money saved for a nice dinner. It seems like you can get the Grant Hyatt for half the price using a stay certificate. However, if one wanted a superb suite and didn't mind a somewhat inconvenient location I would recommend this hotel with a suite upgrade award. To get a confirmed suite in advance for ~$450 a night in an expensive city like Tokyo is a good deal.