I've watched all six (LOVED all six!) but the main arc as you move forward is that the camp factor ramps up. essentially Ogami Itto, the action star, gets increasingly bad ass. so yeah, personally I loved it, but I was appreciating the camp a lot as it went on.

I've read the whole manga, too. if you dig the themes / content of the early movies, you might dig most of the stuff in the 1st 10 books or so. like the movies, the books increasingly start pushing towards an ultimate resolution and get increasingly filler-ish the closer he gets to Edo. and for the last books in the 20s? siiiiigh, I have NEVER seen so much filler in a story. it's kinda amazing just in that respect, but yeah, if you're not looking to find the amazing ways someone can pump filler to extend a story to sell more units, don't ever read past book 15 or so.