One of the hotels that Regent uses (for the program with the upper level suites) is the Park Hyatt - found that out last year after CLEO. The property itself is very nice in a very exclusive area of the city but inconvenient to touristy sites.

We're going back to Istanbul for one night (pre-cruise on Seabourn Quest) in August. I think we'll stay at the Grand Hyatt this time - bookable for $145 all in (many of you know how to book these "deals" with Hyatt based on our past interaction - particularly at the Grand Hyatt Dubai and Park Hyatt Sydney).