I did read the previous reviews prior to our trip to Japan ( may 11th to 24th), and we werent disapointed...
There is not much to add, since everything had already been said...
Although we were asigned "old" rooms for the first part of our trip (4 nights), wich did felt like a hidden trick, since a serious renovation had to be done. But on our return from a Kyoto base camp, they located us in the higher renovated floors, and was well worth the money.
The reception did keep half of our lugages during our 7 days exploring Kyoto, without any questions.
The location is extremely convenient, and close to vibrant districts of Shinjuku/Kabukicho, with enless bars, insane neons ads, etc...Although, to go directy to Kabukicho, dont bother following the plan given by the reception...just walk across the buildings (north-west) and youl be there in 5 minutes!

The Shinjuku station is almost connected to the Hotel, so just a 10 minute walk, and youll find yourself inside the train...

Plenty of restaurant to choose from, etc...

Finally, although it felt like a highly business-oriented atmosphere, our stay was very well chosen!

Keio Plaza Hotel is one of the best hotel in Japan. I stayed in Keio Plaza Hotel Shinjuku last year. It was a wonderful experience. Most important is its location. It is just near to the japan's famous electronic market. Ambiance is very good. Lounge facility is excellent.

Keio is a really nice hotel, it is HUGE and right next to the Tokyo Metropolitan Govt Tower so is very easy to find. There is even an underground walkway directly from Shinjuku station that takes about five minutes. We stayed here on our last night in Tokyo so that we could have a comfy bed (instead of futons) and they upgraded us which was lovely.

We booked with a tour package and stayed in the, I assume, basic rooms. Rooms were clean, basic, and standard size (small!). Check in staff was very efficient and professional. For the price that we paid (around $150 for a double room a night, including breakfast), I think it's very good value.
As for the location, I think it's great. It's really not a tough walk from the Shinjuku N'EX Station, about 10-15 minute. If you plan to go this route instead of the Airport Limo Bus, check online for specific walking instructions. Of course, if you have a lot of suitcases, do the bus. Shinjuku nightlife is a nice walk from the hotel. If you're comfortable with the metro system, there are a couple of lines close to the hotel.
It's definitely not the nicest hotels in terms of luxury or facilities, but if the rate is good, I think it's nice...