They have set up the site so that the most frequently used problems rise to the top of the list. Problems can be exported to a Word or Google document. Have a great problem to contribute? Teachers can upload to Opus too. I think this tool would be great for both pre- and post assessments.
CC Connections: Opus is specifically designed to help teachers address the CCSS in math. They appear to be purely computation problems at this point, very useful for practice or for assessment.