The idea of these United Way tax sites is so simple: Provide free tax preparation services throughout Seattle. Put money back in the community, where it came from. And, as I recall, my money didn't start at H&R Block.
I work in the building that hosted one of these sites, and I was consistently impressed with the professionalism and kindness of the volunteers staffing it. When I had my own taxes done there I felt absolutely confident in the accuracy of the return.
One word of caution: Although the program doesn't have an income guideline, they seemed best equiped to handle simple returns. I would leave stocks and bonds to the people who get paid to spend hours going over them.