Very old and gloomy Marriott in need of updating. The only reason to stay here is its proximity to the airport. And even then, beware.
There are more people trying to get to the airport than there are available shuttles. You are jammed in to the point of panic, where some people were pushing their way, through standing room only, to get OFF. Leave yourself at least 45 minutes to get to the airport because you'll have to wait for the shuttle and then you'll have to wait for your drop-off and then you'll have a good walk from the drop-off to your airline check-in counter.
Expect an extremely long wait to check in. The staff are harried and overworked, and it shows. Don't expect them to know anything about a breakfast coupon (Gold, Platinum members) should the Concierge Lounge be closed, because they don't.
My next flight out of Seattle Airport will include a stay at another brand.