A great alternative to Times Square hotels
I've been to this hotel twice, once for business and once for leisure. 
The location itself is relatively convenient. There are no subway station in the immediate proximity of the hotel, so you need to walk about 5 minutes to Grand Central to take the 7 to Times Square or the 6 to Union Square. Walking to Times Square takes about 15-20 minutes.
The rooms are bland, standard, and reliable. Just what you would expect in a Marriott of this category. The rooms are impecable and a pretty decent size for New York.
Breakfast was good (but VERY expensive), bundle breakfast with your room if you're going to have it, much better value usually. The lunch I had there was catered, but was surprisingly good, especially considering the amount of people that were there. Nothing fancy, but it was obvious they used good quality ingredients.
It might be a matter of taste, but I find both the interior and exterior of the building beautiful and well integrated with the neighborhood. It's definitely a plus for me, especially since it was recently renovated. The neighborhood itself is great. Grand Central is definitely worth seeing. Very close to the Rockefeller Center too.
This hotel often offers deals that are really good value especially if you pre-pay. It's obviously never going to be the same as being in Times Square, but if you can score one of these deals, it might tip the balance because there can be a huge price differential.
I can't say much about the service other than both my stays were trouble free. I had no reason to interact with the staff, but the few interactions that I had were pleasant.
All in all, I think this is a fantastic option if you are going to New York. If this is your first time in New York and you are a little apprehensive about taking the subway or missing out on the action, then it might make sense to pay extra to be in Times Square, but I think it's hard to go wrong with this hotel, especially if you can score a decent deal. It's a Marriott, so don't expect that boutique feel, but I think the beauty of the building somewhat makes up for that. Definitely high on my list for my next trip.