Excellent Location
Stayed six nights.
Pros:
*Location: 
Easy access to 51st street subway station with the 4,5,6 trains. If you need to go uptown or downtown on the west side, just take the train one stop down to Grand Central and use the S (shuttle) to Times Square and transfer to another train. Just be careful not to exit the subway station during these transfers (it's easy to just follow everyone else out the exit door without thinking).
Easy walking to Times Square and Central Park South (both about 15 minutes).
Starbucks on the next block, Red Stone Pizzaria across from Starbucks, and Cafe Metro (pasta, salad, subs) a few blocks away.
*Quiet: Our room, 1517, was away from the street and extremely quiet. Three small windows with views of the sides and roofs of buildings was the trade-off.
*Service: 
Front desk always friendly and helpful. We requested a rollaway bed and we were given the larger 2 double bed room to accommodate it. Again, no view in this room and it was subject to availability.
Jimmy, the doorman, also friendly and helpful. There were plenty of taxis continuously coming by and Jimmy would hail one quickly. The hotel also had cars and SUV's sitting out front for guests to rent. I took an SUV back to the airport so that we would have AC and more luggage room. The cost was $60 + tolls (the driver gave me a choice of bridge with no toll or tunnel with $6.50 toll). This was only $15 more than a taxi. 
*Room: Beds and pillows were comfortable (too soft for my taste, but most people prefer this). Shades were adequate to block morning sun. Closet with full-sized door mirror, disposable slippers, wooden hangers, ironing board & iron, free safe, and blow dryer. Flat screen TV. Coffee maker. Desk with comfortable chair. We also had two arm chairs, an end table, and one ottoman. The AC was adequate. The bathroom in our room was large with vanity inside.
Our traveling companions had the 16th Floor Suite, which was large and beautiful and inlcuded amazing views from the wraparound deck.
*Business Center: Several computers and free access to internet for 10 minutes to print boarding passes, but you have to click through the link on the home screen (one person in our party didn't see it and paid $16.95 to use the internet and then the browser didn't have the correct download to print the boarding pass, so be careful not to do it that way). Printing when going through the link was also free.
*Gift Shop: $3 for a cup of coffee and she makes it for you in an expresso/cappuccino machine. Plenty of NYC souvenirs, but you will pay for the convenience.
*Elevators: Two sets of four that access different floors. Small, but fast service.
*Meeting Facilities: We did not use these, but saw a Bloomingdale's meeting with a Gucci step and repeat. Appeared to have very attentive staff.
*On-Site Restaurant: I had the breakfast buffet once during the stay, and the food was great. Plenty of choices.
Cons:
*Internet Fee: $16.95 per day and it is from noon to noon, regardless of when you first access it. I called and compained and no charge was on my bill, which I very much appreciated, since I only used it for a few hours. This policy is definitely something that needs to be changed!
*No Pool: But this is typical in NYC. The trade-off is the character of the older buildings.
*No Fan in Bathroom: Again, typical in older NYC buildings.
And just a side note about Marriott customer service: I first attempted to book this hotel through a link on TravelZoo, but there was a glitch on Marriott's site that wouldn't let me complete the transaction at the given price. I called Marriott and they actually gave me the room at a better price.