I wrote a similar letter after a disappointing stay at the FS NY. I was careful not to be hyperbolic and to be sure to praise the good things, as well as identify the disappointing things. (I also started with an apology for not writing about all of the wonderful FS experiences I've had.) The result was a phone call from the GM and a letter offering me a free weekend at the hotel (which my parents were thrilled to use on their last trip to NY).