I've read many of Louise Penny's books, but her earlier ones were definitely better than her later ones.
I kinda-sorta agree, though for me I'd say it was the early-middle books that I liked the best - The Brutal Telling, Bury the Dead, How the Light Gets In ( http://www.bookcrossing.com/---/13448081/
). The first book seemed a bit leisurely, and if I'd read it without having already read The Brutal Telling I don't know if I'd have stuck with the series.
The more recent books... well, I agree re Nature of the Beast ( http://www.bookcrossing.com/---/14231723/
), which had some good elements but which felt like it should have been in a completely different series, or a stand-alone. And maybe the author felt so too, as I don't recall any references to it at all in subsequent books, just as if it never happened! (The early imagery was marvelous and chilling, and it was based on actual events, but it was my least favorite of the series so far.)
I did enjoy The Great Reckoning ( https://www.bookcrossing.com/---/14712937/
), though I listed some quibbles re plotholes - at that point I did a lot of shrugging-and-moving-on. Not the best, and not a good starting point, as for me its strength was based on the entire series' worth of character development and relationships.
The Long Way Home ( https://www.bookcrossing.com/---/13448101/
)... not my favorite, but I did enjoy it, at least up until the end, which was... harsh. The road-trip aspect and the ongoing focus on art, artists, artistic temperaments, etc. pleased me.
I've read Glass Houses ( https://www.bookcrossing.com/---/14709934/
), the most recent book, and enjoyed it - though, again, it had its flaws, including a bizarre and unlikely plot-point that was colorful and intriguing but not-quite-Three-Pines-ish.
I still like visiting the characters and locations, and will continue to follow it, but I'd agree that not all the books are equally solid plot-wise, and while there are some that I'd re-read gladly, that's not true of them all!