I read "Showdown" in my 20s and the whole time said/thought "This is a really weird book." I just kept repeating that over and over. I don't think I was quite ready for it. Then, in my 30s, I picked up "Sinner" (without knowing it was a sequel) and LOVED it. Suddenly all the stuff in Showdown made a lot more sense, and I think I had turned a corner in my life because all his books really spoke to me on a deep level.
Oddly enough, I never read the Circle series! I did check out Green once, but not the others.
The Outlaw series is fantastic (though the first one is very different than many of his other books I've read) and if you didn't check out the Mortals series yet, that's great too. He certainly has had a few books that weren't my cup of tea, but mostly he's one of my favorite authors. The way he describes scenes of Glory is as close to what I've seen of Glory as I can put into words. It's amazing stuff.