I stayed here for one night on a business trip. I travelled from Heathrow using the piccadilly line (honestly, it makes no sense to travel any other way if your hotel is on this line) and walked down Cromwell Road to my hotel. I've stayed in a couple of other hotels in this area (the Kensington Rooms and the Holiday Inn) and would rank the Marriott as the best of the three.
The check in staff were very friendly and very thorough. My room was very nice and very clean.
The hotel is right across the road from a supermarket which I used to get something to eat. It's also a short walk to a bigger selection of eating places, opposite the Gloucester Road station.
My only complaint about this hotel is that they, much like many other hotels, charge way too much for the internet. You go to the sign in page and enter your name and room number which is all you need to do to connect at several other hotel chains (Hilton being one)...you are then taken to the payment screen where the internet is in the region of £20 a day.
I joined the Marriott rewards scheme upon check in, although I booked the hotel through priceline so they wouldn't count this stay towards my account! It seems to me like it wouldn't be too much of a stretch for them to give you free internet as a rewards member.
I ended up going to the Starbucks at Gloucester Road to have a cup of tea and use some free internet.
Otherwise, the hotel is great.