I stayed at the KKR for 8 nights out of 10 during a recent trip to Japan on a fellowship, so I got to know the hotel quite well. The place was certainly serviceable, and probably 3.5 stars would be more accurate, but a few minor problems keep me from bumping it up to the elusive four star "very good" category.
The location of the hotel is definitely a plus. It is located right near the Imperial Palace, and indeed from the restaurant on the 12th floor you can look right into the gardens. Definitely a nice view, and a great place to go walking/jogging in the morning. Located right beneath the hotel is the Tokyo Subway (Tozai line) so that you can easily hop on the metro and ride around town. There is a food court type area located around the hotel, and it's walking distance from nice restaurants (and karaoke, if that's your thing).
The staff was friendly and helpful. In another review someone complained about the English, but I (as someone who speaks no Japanese) was always able to find an employee to help me. The concierge staff was particularly pleasant, helping with everything from locating an ATM to finding out information on a baseball game to changing money.
Breakfast at the Fuyo restaurant was nice, although very repetitive. I think a little variety in the selection would have been nice, although I suppose they don't generally anticipate guests staying there for eight days at a time. Nonetheless, if you have extended business to do at the hotel, keep that in mind.
I found rooms to be somewhat small, although certainly workable. I would not, however, want to spend a lot of time inside. The beds were pretty hard; I normally like a more rigid mattress, but this was too much even for me.
One major problem was the lack of good Wifi, particularly on the 15th floor. For the first part of my trip (I checked out for two nights then checked back in) I was on the top floor and shut out of Wifi. I had to go all the way down to the lobby in order to get a signal, and that one was unsecured. My colleagues below me on the 14th floor had a signal. When I checked back in and was given a room on the 13th, I had a signal. It appears, therefore, that the 15th floor is a dead zone. This hotel seems to cater more as a business hotel (given room size and what-not, I'd struggle to use this as a tourist point) and therefore reliable Wifi seems extremely important. I think the hotel really needs to address this issue.
Overall, I did enjoy my stay here. Staff was nice, location pretty good, and food okay (while repetitive). However, after a few nights, you begin to pick up on a bunch of things that could be better.