As many others have commented, the heroine is very typical - whiny, lazy, and a little oblivious (or stupid, if you look at it that way). Not to mention always hungry and having people tell her to go on a diet. However, I, personally, like her anyway, despite all that. Yes, she's the basic main character, but that type of heroine is basic for a reason! One of the most popular manga of all time was Sailor Moon, who's Usagi is almost an exact clone of this Usagi! I understand people may get tired of reading the same character over and over, but I think it's worth it.
And, even if you don't agree with my views on Usagi, the rest of the cast is incredible as well. Mamezo is actually my favorite - he's just too adorable! I mean, a little boy with a bob-cat tail who's trying to protect a girl about eight years older then him? Tell me that's not 'awwww' material! And, of course, Hanzo is just awesome. His character development is fantastic, you can really see the way he changes along the whole journey from when you first meet him.
The plot, I think, is very, very good. There are almost constant twists and turns, and a cliffhanger at the end of pretty much every chapter. The history subtly woven in is interesting, and makes the story that much better. Small, seemingly meaningless things mentioned in, say, the second or third chapter, may turn out to be extremely important maybe the twentieth chapter. Then you look at it and think, "Oh! I remember that!", and feel that much more drawn in. I love it. And the action-filled ninja bits certainly up the thrill factor, turning this series into something much more then your typical shoujo.
Despite the fact that there haven't been any updates in almost seven whole months, I really love this manga. I'd recommend it to anyone.