I had IBS-D about as bad a D gets. It took me about a week on Heather's diet to start to feel some relief, three months to feel darn good, and 6 months to feel pretty much normal. I finally felt so great, I started eating way too far off the Diet and set myself back, so I definitely agree with the "don't cheat" advice.
I took Acacia as my SFS. Heather's diet helped with the cramps and horrible urgency, but I think the Acacia helped a lot with frequency. I started with 1/4 tsp of Acacia per day and worked my way up to 8 teaspoons a day and lots of fluid.
Immodium is a definite help and I have Donnatol for bad attacks. (I hadn't had a bad attack in months before I fell off the wagon.)
I would say absolutely try the Diet and try it straight down the line. It's well worth it.
-------------------- [Research tells us fourteen out of any ten individuals likes chocolate. - Sandra Boynton]