Philip K. Dick was always better at coming up with good ideas than executing them well. He had a brilliant imagination, but in general the only thing duller than his characters is his prose. That's perhaps a bit harsh, but I just don't think he's a great writer in stylistic terms.
It depends on the book. I think it just took him a long time to get the hang of it. What you've said is definitely true about his early books, though. With the exception of
The Man in the High Castle, those early books have good ideas but the characters are pulp sci-fi cardboard cutouts and the prose is mediocre. There's a big change starting with
A Scanner Darkly, though. The writing is better and the characters have much, much more depth.
The Transmigration of Timothy Archer was almost nothing
but character and it still managed to be a very good book. It's a shame he died so young, just as he was really getting the hang of his craft. I'm sure his next books would have been incredible.
Also, I haven't read American Gods, but I hear it's a very divisive novel, even among Gaimen's fans. Out of interest Daniel, have you read Tanith Lee's Flat Earth series? I've been thinking about picking it up for ages, and you seem like the kind of person who might (for obvious reasons) have read it!
I haven't read any of that. I don't know much about them, but they always struck me as being very wizards/dragons-oriented, which isn't really my thing. I read a fair amount of that as a kid and haven't had much desire to return to it recently. Maybe some day, though!