I am a fan of Jan Fennell for the most part. When my dogs started fighting, I was VERY into her methods. Of course, I was grabbing hold of anything that I thought might help!
I don't buy into everything she's selling about alpha behavior, etc. We are the alpha by default because we control the food. After working with a behaviorist for several months, I tend to agree with focusing on behaviors, not on this alpha stuff that we may or may not really understand. I think her method works for her because she's a natural leader. She is trying to teach people how to be the leader of their pack through ritualized behaviors that alpha wolves display, but realistically, you can't fake it. To make her methods work, in my opinion, you have to take on an alpha personality, not just mimic the behaviors.
However, I have found that Jan's method has been quite effective in making comings and goings non-eventful for my Conner who has severe separation anxiety. I don't sit and ignore the dog till the egg timer goes off, but I do find things to do when if I first get home like reading the mail, talking on the phone, etc. that doesn't get my dogs all crazy.
I do think the book is great and there are some parts of her method that are effective in helping certain situations.
But, I actually prefer Jean Donaldson's perspectives. She's more focused on the dog's behavior and reinforcing the behaviors that are wanted positively. She's a bit high brow and her books aren't as easy to read, but if you can get passed that, I do prefer her books. The pamphlets she's published (Fight! and Mine!) are way too high level for the average dog owner, so I don't recommend them, but The Culture Clash is great.
I admit that I am a bit biased. My behaviorist was trained by Jean and my dogs now get along.
