The 6th grade book reviews all the material learned in the earlier grade. Don't bother with the grade 1 or 2 workbooks, just teach a few of the parts of speech then. With Shurley they'll pick quickly unlike the other programs that drag on day after day, year after year. As someone else said, with Shurley the child really owns the material. We've used IEW for 3 years now and my children's writing is superior to the other kids their age and they no longer roll their eyes at having to write silly essays.
Writing is something they ASK to do now.
Unfortunately, I disagree with most of Susan's recommendations. While her comments about the Classical Approach are interesting, they aren't anything new. You would be doing yourself a favor to read other material on the classical approach in order to differentiate what is truly the classical method and what is the authors of TWTM preferences. The kids learn quickly and effortlessly. You may spend more concentrated time when a new idea is introduced, but after that the kids can easily work alone for several days or more depending on the unit.