A bit of history on this. I bought the first three volumes of this series in early 2003. I was impressed with pretty much everything about them. However, it was a bit hard to follow (complex story), especially back then, and, well, too graphic. However, I wanted to finish the series at some point.
Last year (2009), when I was in Japan, I saw a whole bunch of them in the used book store and since the shipping cost becomes null with me carrying them home, I bought everything they had (volumes 4 to 13). I saw more volumes in the new books section, but I passed for the time being. Even with just the used books I bought, I set my return trip luggage weight record.
I went to Japan again at the end of 2009 (returning early 2010), and this time, I bought 14-18, new. Volume 18 turned out to be the last volume.
It's far too complicated for me to describe this series. And since there are both Japanese & English wikipedia pages, so I'll just let you utilize your favorite on-line search tool. Apparently, Dark Horse Comics has released English version upto volume 12, so if you can't read Japanese, English version is an option. There are 6 more volumes to go, though.
