Nice room, tiny bathroom
There are two Marriott hotels in Union Square. This stay was at the one on the corner of Sutter and Stockton STs.
The hotel is a nice property with doormen opening the doors as you enter and exit. Staff was friendly and attentive.
The lobby isn't much: basic space to support check-in, check-out, a bell desk, and access to a restaurant.
The rooms are comfortable with a cushy bed, flat-screen tv, desk, and great views (if you are on a high floor). Space is a bit tight. The room had a feeling of a long, narrow galley. I was disappointed that there wasn't a place (other than the desk) inside the room to eat. 
The closet was also small. Rather than a bar across the entire closet, there were to bars running from front to back. This allowed the closet to be more narrow. This might work well for people with roller bags. It was a nuisance for me because I have a hanging bag. Once the bag was suspended across the two bars, hanging clothes had to either be hidden behind my bag or placed in front, limiting access to the bag. Tolerable, but annoying.
Most frustrating was the size of the sink in the bathroom. It was tiny with limited space for tools (hair brushes, shavers, toothbrush/toothpaste, etc.). And using it I felt cramped by the shower curtain on a curved rod that caused it to insinuate itself into the space for the sink. I couldn't get it out of the way enough to not feel cramped as a shaved or brushed my teeth.
Maid service was attentive. And, as a frequent stayer, I received nightly turn-down service: closing the curtains, a brief tidying up of the room, and leaving two chocolates on my pillow.
My Marriott status also entitled me to the use of the Concierge Lounge. The lounge served a breakfast of oatmeal, cold cereal, scrambled eggs, the usual unripe fruit (cantaloupe, honeydew, and pineapple), and good pastries. The croissant were crispy and flaky. There were scones, pain au chocolate, and various fresh sweet rolls. Orange juice was served at breakfast. 
Coffee, and a selection of teas, soda, and beer are available throughout the day.
The afternoon snacks were disappointing. Turkey and ham sandwiches on focaccia, a hot appetizer (e.g., mini vegetable spring rolls that were mostly fried dough with limited vegetable filling), a tiny vegetable "salad", and a cold antipasto tray that usually included slices of sausage, cheese, and olives. A later dessert included an assortment of three mediocre cakes and brownies.
A nice feature is that the lounge is open all day even when food is not being officially served. Soda, coffee, and tea remain available. Between meals, mixed nuts, pretzels, and freshly baked cookies are also available.