I really do. Here's why.
Art: Top notch. The best 'realistic manga' styles I've seen in a while. On par in its own way with Takehiko Inoue's art, who I consider to be King of this style.
Plot: Very interesting. Our leading man, Manji, was given immortality through regeneration, wants to be mortal again, and so decides to kill 1000 bad guys to compensate for the 100 innocents he's killed. Our leading lady, Rin, seeks revenge for the senseless murder or her parents and annihilation of her dojo's sword style by the Ittou-ryou school, and enlists Manji as a bodyguard in order to do this. They proceed to fight members of the Ittou-ryou and come across other factions and individuals with different and/or similar goals and interesting things happen. One great strength is how none of the opposing sides are definitely 'villains'. They all have their individual reasons for doing what they do, and these motivations are what bring them to conflict rather than 'good guy vs bad guy'. The occasional twists are nice too. Fairly simple overall, though the plot is laid out rather basically from the first few chapters.
Characters: Fantastic. As I mentioned before, one of the strengths is that very few of these characters are definitely 'good guys' or 'bad guys'. There are a few exceptions of course, and they are dealt with accordingly. The character interactions are always good, a surprising number of romances/couples as well. One small criticism about Manji is that he barely changes, though this serves as a foil to Rin's development and possibly serves to remind us of his immortal and unchanging body. The fact that the main antagonists (almost all of them really) are characters you get to know rather intimately is a good way of making sure that you don't see them as black and while bad/good guys. Which is one of the themes of the manga, it's not a mindless killy killy samurai manga, it has heart. Rin's struggles over her quest for revenge, Manji's reluctance to define who is 'evil' so he can kill a thousand evil men as well as the overlaying theme of forgiveness, redemption and all that good stuff.
This isn't a macho-fest full of dudes playing with swords while the girls bandage them up afterwards, by the way. Arguably the strongest fighter in the manga is female, there are several strong female characters and fighters, and the point at which Rin springs Manji from jail is one of my favourite moments in manga.
Battles: Top Notch. Beautifully drawn, certainly gets better the further in and even the fact that Manji is an immortal doesn't retract from the tension. His immortality is not a Super-Sayian-like invincibility booster, there are still ways to kill him, disable him and defeat him, but the regeneration makes it a lot more work than it would have been. It certainly isn't a 'seen one swordfight, seen them all' scenario either, there are a diverse cast of warriors each with different styles and often completely varied (and often fictional) weapons, and this leads to a battle manga that stays fresh.
Offpoint: One common criticism is that Manji relies to heavily on his regeneration and not his actual skill. I'd like to point out that in battle, your basic reaction and the way you move ultimately stems from your instinctive fear of death. Manji still feels pain, but he doesn't really have a life-or-death reason to block a sword thrust to the gut anymore, I would argue that his regenerative abilities have pampered him and made hims somewhat soft - and that this is why he takes so many hits when he's supposed to be skilled.
And a little warning.
Maturity/gore: This is a seinen for a reason. The fact that it's a samurai manga should be caution enough, much heads being lopped off, dismembered limbs, geysers of blood (though not to the point of ridiculousness) and later, it gets worse. There's this one character who is definitely one of the 100% evil characters. He rapes girls while cutting them up and you get to see this. There is also a prolonged rape/interrogation of one of the characters, and an arc more or less given over to chopping people up for experiments. This is generally handled much better and is not as bad as some other seinen (Wolf Guy, Hellsing, Freezing, Berserk to name a few) but if you really, really don't want to go through these scenes, don't begin reading.
Overall one of my favourite and most well-balanced manga I've read, once I've finished it, I'll go take a gander at some of the mangaka's other work.