I stayed at the Ace Hotel for work and overall I think it is a good place and would stay there again but not everybody would like it. The hotel is decor is somewhat IKEA sheik but if you are into contemporary boutique hotels you will like this place. The location is great and I could walk everywhere I wanted to go and there are more than enough places to eat and drink in the area. I did not get a chance to do much sight seeing but the Public Market, Space Needle, Chihuly Garden and downtown are an easy 1/2 mile walk.
Pros are The staff was more than friendly, the bed was very comfortable, and everything was clean. Continental breakfast was included and the Ace is located in the middle of just about anything you would want to do in Seattle. It was also affordable at $99/night. Street parking is metered except for Sunday and there is a public lot that is available to park in. The Cyclops tavern is in the same building and was a great place to grab a beer and get lunch.
Cons are I stayed in a inexpensive room on the courtyard which shares shower facilities with other guests, and I admit it was a bit weird for me. It is like sharing a bathroom at home so bring flip flops. If you really don't want to do this there are larger rooms with private showers available too for around $60 more per night. The room was also a bit small but I mostly just slept there. The only other con was on Friday night it was a bit noisy as the building is old and if you ever lived in an old apartment building with hardwood you know what I mean. It was never really loud but if you are easily awakened then stay at the Marriott instead.