We stayed here for one night two weeks ago pre-cruise. Last year, we stayed at the Park Hyatt one night post-cruise.
While we were very happy with the Park Hyatt last year, we chose the Grand Hyatt this year based on cost (Hyatt Daily rates are about 100 Euros lower at this property or 3000 points less since it is one category lower). However, I booked using a stay certificate. The property is also more "centrally" located near Taksim Square rather than the high end boutique shopping area near the Park Hyatt.
Upon arrival at the property, we went to the front desk to check-in. When the agent noticed that I was Diamond and that we're to be in GC room, we were escorted to the elevator to finish checking in at the GC (which is on 8 along with 9 are keyed off).
We were assigned to 847 (on the front of the building on the left if you are facing the building) - rooms on that end have some view of the Bosphorus that is not blocked by the monstrosity of a building which is the Ritz Carlton. 847 appears to be a standard King bedded room. Upon entry into the room is a closet then the bathroom with a separate shower (a little bit too small) and tub, the toilet is in a separate area but there is no door to it. Then the standard bed area with a credenza and flat panel TV opposite then a desk with two chairs and a chaise lounge. Overall, a perfectly comfortable but unremarkable room.
The Grand Club is a spacious area on the 8th floor (during service hours includes the two "conference rooms" at either end of the lounge). The GC offerings are rather "skimpy" though - a little bit more than the standard domestic Hyatt Club lounge (includes alcohol) and not nearly as extensive as those in Asia. The evening service certainly does not really make a supper. The GC is probably somewhat overstaffed since the staff is a bit too attentive and made us a bit uncomfortable.