I have not been to one of these in particular, but I did have the opportunity to go to a Jamie Abersold workshop a couple years back in at the UNC jazz festival in Greely, CO. The workshop was great, even though it was geared mostly for beginner soloists. He does a lot with scales and teaching the relationships of chords and scales. A really great teacher. I would assume all of the scheduled instructors would show up to the seminars. You will learn a lot.