Just finished 'TGOS', and thought it was a good but not great read.
The pacing was a bit off in places, too slow at first, then it felt like it was one thing piled on top of another! The basic story of Ellen Preddle was compelling, but I found the story lost its impact as more characters were brought in. Plus, it felt like Herbert was repeating himself a bit near the end (those who've read 'The Fog' will understand). Not the firs time I've felt a JH book could have been about 100 pages shorter with no harm to the story.
Maybe I got spoiled by reading the not mega-long 'The Rats'.
As for David Ash...I was interested to see what more we'd learn about him, but now I feel like his story has run its course.
I'm not sure I'd be too enthusiastic to read a third story about him, though I like the Psychical Research Institute.
If they could keep the institute, but find a sensible way to write him out, it could work.