CM writing is very informal, as far as instruction goes. I have chosen a middle ground – a moderate writing instruction program along with oral/written narration. That has worked very well for us, and my kids are loving writing and realy improving.
We use Aplogia’s, “Jump In” and my kids absolutely love it – but there are really a lot of good instructional programs out there.
My concern with IEW is the ‘formulaic’ writing approach, which was discussed on this thread:
http://simplycharlottemason.com/scmforum/topic/andrew-pudewa-for-what-its-worth/page/2#post-119841
I have never used IEW, though, so I have no real opinion there. People seem to love it or hate it :). Like so many things in Language Arts…it seems that taste is subjective!
Hope you find what you are looking for out there in homeschool world. Happy writing!