Here are my ratings for some of the Hillerman books I've read:
The Great Taos Bank Robbery--7.5
The Dark Wind--8.0
The Wailing Wind--7.5
The Sinister Pig--2.5
Sorry, but that last book sucked. After a few chapters I had to give it up.
I've read more of Hillerman's books--e.g., The First Eagle, Talking God, A Thief of Time--but that was before I created my database of ratings. I'd have to reread them to be sure, but I think they were 7.5s or 8.0s.
Hillerman's books are usually in the 7.5-8.0 range, which means good or very good but not great. The plots are ingenious, as noted previously, but too complex. He ususally juggles 10-11 characters in a web of connections, and you need a flow chart to keep track of them. Following more than seven characters is often too difficult.
For more on the subject, see Tony Hillerman Dies and The Best Indian Books.