Having only read the first three books, I have to gauge continuing by what I've read in reviews.
My overall impressions:
Books 1 to 3 are great.
Starting around book 5 reviews turn mixed.
By the time you get to book 7, 8, and 10, it's excruciating. (I've heard a few times that book 9 is better than the ones before and after it.)
I found the Crossroads of Twilight page on Amazon funny because it had over a thousand reviews and was averaging one star.
I'll note your take on books 4-6 being good.
I haven't read the newest two books of course, but I did read the author's first stand-alone novel and it was not amazing but reasonably good.