The W is closer to Sultanhamet (the old city) -- where the Blue Mosque, Aya Sofia, and Covered Bazaar are -- than are hotels in the business districts, but that doesn't mean it's close. It's on the Bosphorous, near the Kempinski and the new Four Seasons (supposedly opening in July). In my opinion, it's the best location in Istanbul, particularly in summer -- near Besiktas and Ortakoy and down the hill from Taksim. I have walked from there to Sultanhamet, but it's probably an hour. There's also the light-rail connection to get you part of the way there.

So it depends where you want to be. Most first-time visitors want to be in Sultanhamet, in which case the Four Seasons is the ideal location and the W is not. The two Hiltons, especially the Hilton Istanbul, are slightly closer to Sultanhamet than is the W, but not much. But if you plan to be in Istanbul for several days and plan to see the whole city, I would choose the W location. I can't wait to check it out.