The venerable Stephen Gallagher says on his blog: "The downside is that there are a lot of books, courses and how-to's out there that ain't worth a bucket of warm spit." Which is very difficult to argue with but then he says "when somebody's telling you stuff about writing, check out what they've written. " Which is very easy to argue with as I've learnt loads from critics for a start.
Mr Gallagher goes on to plug William Akers course/book promotion on Thursday. If, before buying the book or taking a ticket, I decided to check what Akers had written, here on IMDB, I wouldn't give him the time of day.
However going on to his website, (check out the free handout) I find that he's been a script reader for 20 years. Writing Ernest Rides Again (or Citizen Kane for that matter) is no guarantee that a screenwriter is going to tell us better stuff about writing. Reading and analysing scripts for 20 years isn't necessarily a guarantee either but it's much more likely. Now he gets the time of day.
Writing and teaching writing are two different skillsets. Of course some can do both well but let's reserve judgement for those who can't do either well.
Sir Danny did a Twitter survey yesterday on the best screenwriting books asking for the one book writers would choose. The results are here. Only William Goldman and Russell T Davies, are famous as writers. (Although someone, being contrary, lists a website, Wordplay by Ted Elliott and Terry Rossio.)
Few, if any, of the books were named twice, which proves something, I'm not sure what. Perhaps not that we should buy every book and do every course until we find a method that suits us. I've known writers who have done that but that's the fear talking.