I love this book.  I've found it incredibly useful and full of lovely little bits of trivia - as long as you don't take it as holy writ.

However, I'm told he's not that great on the "how not to poison yourself" aspect of herbal writing.  I don't have any specific instances, but everytime I mention an affection for this book someone else will come along with this warning.  

I'd definately recommend doing what I always do - before ingesting (or inhaling the smoke from) any herbs on the recommendation of a single author, do thorough research in a good medicinal herbals first.