I stayed at the hotel in December 2017 and wanted to add my comments.
I am a plat and was upgraded to a suite overlooking the "beach" as someone put it. As others noted, the quality of the bedding, towels, etc. is great.
It is now a Cat 3, but still a great use of points; they had C&P availability, which made it an even better deal.
I agree that the restaurant is over-priced, but the breakfast and the evening snacks in the lounge are great - I felt like skipping dinner. Not an infinite number of choices, but enough to make a meal of it if you want.
The lounge is closed over weekends; you get to eat the buffet breakfast in the restaurant instead; there is no make-up for the evening food.
Several times I had to leave the hotel early in the morning before the buffet started, and the hotel was happy to pack me a box of food.
Outside the hotel, where cars pull up, there is a plaque that indicates that the president of Guyana opened the hotel -- that might explain why there is a Marriott here and not a Courtyard.
I agree that the architecture of the hotel doesn't quite jibe with Georgetown, but then again, it is not smack-dab in the city, but a few blocks north. The Courtyard in Paramaribo, though, did at least style its roof to look a bit like the colonial architecture.
Re cash, every taxi driver took USD, as do the airlines. Some shops in Georgetown only accept local currency, but you can find money-changers fairly easily.
I can't imagine I will be back in Guyana, but if I were, I would look forward to staying at the Marriott again.