I have been staying there because they have been upgrading me to a suite every time. But as I noted above, my last two experiences were less than ideal, and the continued lack of response to my email is very troubling.
Also, one thing the Hyatt has going for it over the 4S is location. Portman Square is way better than Canary Wharf.
Currently, London hotels give the least value for money of any major city in the world (except possibly for Moscow, which I don't visit). In other words, you get less hotel for your pound in London than anywhere else, especially if you are getting those pounds by converting USD.