Come to think of it, I really don't like grading--at all, under any circumstances, ever. However, since Steve insists on this necessary evil, here are some of my thoughts on the issue.
While I've never tried portfolio grading in C0150, it has worked well for me in C0250 and C0301. Over the years, I've experimented with various assignments that culminated in a number of portfolios over the course of a semester. Currently, I'm using a system that combines traditional and portfolio grading to accommodate not only some of my students' needs but my own as well.
Obviously, there are disadvantages to traditional grading: a) Students are tempted to write for a grade. Once they have that grade, they're stuck--for better or worse. A student who receives an 'A' may rest on his or her laurels for the rest of the semester and not grow much as a writer. Worse yet, a student who fails the first assignments will be discouraged for the rest of the semester, hate writing. . . and my guts. What's a good teacher to do? b) Traditional grading can also lead to choppy assignments. I like to work with sequences that culminate in a major project, so it's difficult to smack a "grade" on bits and pieces of the process.
Alas, portfolio grading does not solve all problems, either. a)No matter how well I explain the concept (in writing and rhetoric), some students don't understand that this is a GIFT. Every time I collect intervention drafts, for instance, a couple of dodos will say, "I didn't put much work into this because you're not grading it anyway." Grrrr! Most of the time, this is more of a self-discipline/time-management problem on the part of the student rather than a problem with the system. b) Of course, instructors are not immune to time-management problems, either. Depending on the required content of a portfolio, how many classes/preps I happen to have that semester, and what else is going on in my life, I may have to do some serious juggling of priorities (--builds character).
Since both traditional and portfolio grading have advantages and disadvantages, I use a combination that's comfortable and manageable for me. In C0250 (Writing Arguments), for example, I assign traditional grades for about a third of the semester--with option to rewrite for students who struggle (but I don't advertise this in advance). By the time we start portfolios, we have established a learning routine and students are pretty clear on my expectations and standards. Now they can concentrate on writing without worrying about grades for every little chunk of work they do. In addition, students have more control over what gets done and when--as long as it does get done. At the beginning of a given unit, I distribute a check-sheet for work due at the end of the unit, so there's no ambiguity. The content of a portfolio is determined by the instructor. (Do you prefer several smaller portfolios--or a couple of more extensive ones?)