What is the role and format of the qualifying exam in CS/ECE programs where the exam is not in the form of a comprehensive written exam?
Where I did my Ph.D., we effectively had no written qualifying exam.  Instead, students were expected to gather high marks following a certain complex distribution of classes within the first two years (there was also another component, involving giving a research talk, but that was more equivalent to defending a thesis proposal).  If you could not achieve enough high marks, then you had to fall back on a more traditional qualifier for whatever subject areas you couldn't pass out of via classes. I never knew anyone who didn't just get the high marks, and the rumor amongst the students was that if you had to use the fall-back, the professors were sure to fail you because they'd be grumpy that they had to prepare a whole exam on your behalf.

The rationale for this method comes from the department's goal for the qualifying exam: ensuring that students have a sufficiently strong grounding across a sufficiently broad area of expertise.  Given this goal, why can't high marks in key graduate-level classes establish expertise just as well as a special non-class exam?  Adding a special exam just increases the stress on students and the burden of preparation on faculty.

As a student, it was certainly much appreciated---if you're in a Ph.D. program, you're probably pretty good at classes, and so being able to tackle the whole qualifier thing just by being good at classes was quite a relief, especially compared to the stress that people I knew were going through elsewhere.  I think the faculty really liked it too, because they could deal with the whole qualifier thing just by teaching the classes they wanted to teach anyway.

As for down-sides... I suppose it means the faculty have to trust each other enough to believe that high marks actually mean a student is qualified?  Other than that, I really don't see any down-sides.  Personally, I think that the traditional qualifying exam is essentially academic hazing and should be replaced by methods like this.