I stopped here for an overnight stay with family paying with points at relatively short notice (89k from memory, so pricey, but the rest of the city wanted USD1000 for a Courtyard so clearly it was a busy night, and I didn't want to stay in a distant courtyard). As an ambassador, we were upgraded to a Charles River Suite, which really had a spectacular and expansive view of both the river and the city, we were on one of the top floors. There was also a good welcome gift of prosecco and fruit. Considering I hadn't used any NUA I was very happy. The hotel is certainly atmospheric too with the jail set-up and history. Was also happy with the location as we walked into the city centre one day, and then used the metro the day after, very easily. And then towards the end of our stay, after a late check-out at 4pm, we walked along Charles Street to Boston Common, really cute street and obviously a nice park too.
Breakfast was selected as a welcome gift - you were given a coupon for two continental breakfasts from the al-la-carte list (i.e. it is no longer a credit, but rather a specific dish, two of them for two adults), but I would observe that it was a bit restrictive as a breakfast item (e.g. you had to chose between toast and bakery items for example and couldn't have bacon and sausage with your eggs) and we did pay extra for another plate for the family, but you could eat for free.
I'd have no objections to returning, but wouldn't consider it 5*. Service was fine, rooms good, but nothing above and beyond - bearing in mind that our room was in the modern quarter, as I presume many rooms are, so what you have is lovely indoor and outdoor spaces to hang around in.
Hotel Lobby
View from our room of the Charles River
Welcome Prosecco, was really good quality actually
Charles River Suite, Main Bedroom
Charles River Suite, living room (also had a sofa bed)