I am currently staying at the GH for a total of 18 nights. I used suite certificates to get upgraded to a King Suite/Verandah Suite. I thought I'd give a (subjective) update on the GH, focusing on the things that are important to me.
Location was one consideration for me. The hotel is in a relatively new development part of the Santacruz (East) area and I work in Vikhroli (West). It is about a 40 minute cab ride. The Renaissance would be closer but the Hyatt properties looked more appealing. The Grand Hyatt location is also more convenient than the Renaissance if you choose to go to Juhu (beach) or Colaba/South Mumbai (longer commute!). The hotel is close to the airport (as mentioned before) - if you arrive on an international flight after midnight the drive is a short 15-20 minutes. Plan on using more time during the day though.
Walking around the hotel is definitely possible if you would like to; however, the Santacruz area with more restaurants/shops is a 10-15 minute cab ride away.
The room layout is nice, comfortable living room with desk, two-seater couch, two chairs and a coffee table. TV is a Samsung 42 inch flat-screen with 40+ international channels. Bedroom also has a Samsung 42 inch flat-screen TV. The bed is very comfortable with nice crisp soft sheets and several pillows that offer moderate to firm support. One thing to note: the closet/drawer space is relatively small compared to other hotels I have stayed at. Not a problem for me but it might be if you are traveling with your significant other.
The bathroom has a roomy sink area with plenty of space (definitely not a problem when traveling with your significant other!) and a wet area with shower and bathtub. The bathtub is big enough to fully submerge, and bath salts are provided every night. The shower has excellent pressure and temperature.
Floors 1-2 are the Grand Club rooms, the Club lounge is on the first floor but has stairs leading down from the second floor. The lounge is truly comfortable and has not felt crowded during my first eight days here. They serve a nice continental breakfast and a very good cocktail hour (from 6-8pm) with a selection of hot Indian snacks and a variety of bread/cold cuts/ daily changing appetizers (e.g. prawn cocktail, tomato&mozzarella, etc.). They generally have four desserts which are always outstanding. If it's important to you - they have an open bar and will serve you a variety of cocktails from what I could see.
The lounge also has a small business center with copy machine, two desktops with attached printers/scanners. They work well if you want to print documents (like boarding passes) - there is a small fee (I believe 35 Rps per page).
The lobby is truly gorgeous, vast, with great artwork. The restaurants are on one end of the lobby level. I have tried Soma and Celini so far, both offered very good food for reasonable hotel prices. The lobby has a concierge section and a check-in section. Every single employee will be happy to help you - I used the concierge to arrange hotel transportation or a private tour with guide. General check-in will be able to change larger bills into smaller bills (which is generally a good idea if you plan on taking taxi cabs on a regular basis). The hotel will hail a cab (a/c or non-a/c depending on your preference) from the outside for you and, depending on your destination, will determine a flat fare. The fare is slightly higher than your metered fare. I never had to wait more than 2-3 minutes for the taxi to pull up.
The spa/gym/pool entrance is on the ballroom level. The men's section of the spa has a small whirlpool, a sauna, and a steam room (in addition to your locker and shower section). The gym is fairly big and offers modern equipment, definitely sufficient for my basic workout. The pool offers comfortable lounge chairs and sufficient shade.
Overall it is a very nice property for my extended stay. I cannot compare it against other properties but I have found the service here to be outstanding. Staff started greeting me by name after two days, employees are always available and helpful. I have not encountered an incident where my requests have not been fulfilled. I will definitely consider staying here again next time I will be in business in Mumbai.