If you go - and it's well worth it - plan on dining. There's something about dining with a whole view of the city beneath you- boats on the river, planes, etc, that will make this one of your most memorable dining experiences. I thought the food was good and reasonably priced, but that's the secondary experience.
I remember well celebrating an anniversary in the old Windows, and have no recollection of the food, just that it was a memorable experience.