I've been very pleased with my Platinum status over the last 2 years and am crushed to be losing it this year. I can say that I've had very good experiences at the:
Westin Maui (just returned today) - booked 5 nights award stay in a Garden View and paid 2 nights in Mountain View to be upgraded to Deluxe Oceanview (also received additional benefits of 20% off already-high spa prices)
Westin Dublin - upgraded to Presidential Suite overlooking entrance to Trinity College (2 floors - private sauna, fitness room, etc....)
W New Orleans - French Quarter - Stay 10/get 2 weekend nights free - upgraded from basic room to "Carriage House" room with private outside patio and hot tub

I do agree, however that Platinums are pretty common out there and it's especially difficult to get good upgrades in places like some of the popular NYC hotels (No upgrade at the Westin Essex House, but a sweet room overlooking Union Square at the Union Square W).