Nice amenities, great staff, so-so location
I stayed for an extended period of time in a suite, which included two rooms, a good-sized kitchenette and a balcony. 
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WHAT I LIKED
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THE ROOM: The layout was very livable, with just the right stuff you need to lounge, work, and cook a basic dinner. (Also, the alarm clock was easy to use and the bed was comfortable.) The balcony with a lake view was a nice feature.
FACILITY/AMENITIES: The hotel offered a pool, hottub, sauna, exercise room, and laundry room (and they have rolls of quarters at the front desk). There's free breakfast (like, real hot breakfast--not continental) 7 days a week, and free dinner M-W. There's always free coffee in the lobby, and plenty of room to sit. The front desk staff is extremely friendly and professional.
NEARBY: The hotel is across the street from the lake, the Center for Wooden Boats, a marina, a small beach, and a museum. The hotel is also very close to the Amazon campus (if you want to see where the magic happens) and very walkable to the flagship REI store.
FOR DOG OWNERS: This hotel is pet friendly (with a fee - I think $10/day). They have a designated grassy patch around the back of the hotel, with free poop bags. Also, note that dogs are allowed on the SLU Trolley (which stops right in front of the hotel and is free to ride), as well as on city buses (you have to pay an extra fare for the dog). There is a new (usually empty) dog run under I-5 in Eastlake (entrance near Garfield St.) and a (usually populated) dog run at Denny Park--both of which are about a mile away.
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GRIPES 
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THE ROOM: The bedroom TV didn't work. They checked it out 2 or 3 times but never really offered a remedy.
Construction outside made things very noisy. Before opting into a room at the front of the hotel, you might consider whether the Mercer St. Corridor project is finished yet. 
Marriott usually has awesome pillows, but this one didn't. They were just average.
WHAT'S NOT NEARBY: The hotel is not exactly at "the heart of it all" for tourists--a mile from the Space Needle, 1.5 mi from Belltown, and 2 miles from Pike Place Market. (There is a good bus/trolley system, though.)
Also, there aren't a lot of groceries in walking distance from the hotel. There's one small convenience store nearby, but the nearest real grocery store is a bit of a hike.
PARKING: Lastly, as noted by other reviewers, parking at the hotel is expensive ($23/day). However, you can pay for street parking in the neighborhood behind the hotel. (The private lot across the street is even more expensive.)