Also, there is so much flexibility built in to allow the instructor to customize the assignments for individual students that it is actually not as easy as it should be to assign all the problems in a problem set to all students, which I would have thought would be the default behavior.
I assign all problems to all students by clicking on the number in the "edit assigned users" (e.g 1/50 one out of 50 students has been assigned this set) and then clicking the "assign this set to all students button".
Even more flexibility is possible by clicking on the "Instructor tools" link -- the headline over the other instructor tools -- many people miss this one.
On that page, using command/control click, you can assign an arbitrary number of selected sets to an arbitrary number of selected students.
Are there other use cases we should consider?