I haven't taken a workshop from John Coffer but I have signed on to do so for the end of May. I have his "Doer's Guide" and new instructional DVD and both are very informative for someone starting out. The cost of the manual is also credited towards workshop tuition so it is a pretty good investment especially if you are kicking around the idea of taking a workshop.
Coincidentally I also received a letter from him today in which he offered some advice to help solve a problem I've been experiencing lately as I attempt some ambrotypes. He seems very down-to-earth and helpful, and he comes highly recommended by everyone I've ever corresponded with about wetplate collodion.
I'm really looking forward to his workshop. I think it will be very informative and great fun.