It's too long, of course. No story needs 10,000 pages, especially not a shounen manga with demon-fighting, filler stories and the same Big Bad throughout. (It takes Naraku a few books to show up, but thereafter he's there to the end.)
It's far longer even than Rumiko Takahashi's earlier blockbusters, Urusei Yatsura (34 vols.) and Ranma (38 vols.). I assume you've heard of her? If you haven't... well, she's been one of Japan's most successful manga-ka for about forty years now, with multiple hit series. Most of them are comedies. This one's bloodier and darker.
It's good. I'm a fan. However it's clearly too long, even by the standards of long-running manga. Tomoko gave up on it. She reached vol.40 or so. I kept going, but towards the end even I got a bit impatient with all the filler and monsters-of-the-week in what was clearly the closing stretch. (Is that an unfair perception? I had all 56 tankoubon on my shelf, which wouldn't have been true for someone reading it on original publication in Weekly Shounen Sunday.) It's even tempting to argue that criticising its length is a category error. It's a weekly manga that ran for twelve years. No one bashes DC Comics for not having finished Batman yet. Obviously you could tell this story in fewer volumes, but I find something quite soothing in its relaxed pace, character development and bloody yokai horror. It's like gruesome, ultra-violent slice-of-life. It works. I like it. Only towards the end did I start caring about pacing issues, because by then the regular cast had got friendlier and the format was starting to get a bit cosy.
(I also admire the subtle character development and how Takahashi gradually layers it into this long run, so I think you'd lose something by compressing the plot. It's surprisingly hard to draw up a list of the only volumes you really need to read. Stories that look dispensible will actually be important for someone's characterisation.)
The difference between this series and Batman is that this is tight, unified and feels like a single story. It's all about its four heroes and their struggle against Naraku and each other. It really is one 10,000-page narrative. In fact, it's different even from other long-running shounen manga like One Piece, Rurouni Kenshin, etc. Such series tend to reinvent themselves every eight books or so, with new supervillains and story arcs. It keeps them fresh. Admittedly this show does have secondary villains, including Naraku's stooges, sidekicks and severed portions of himself. (The latter can be the most troublesome, since they're liable to get ideas above their station.) Naraku's the kind of villain who seems to have a psychological need to taunt you about how he's immune to your attacks... and then when you eventually prove him wrong, he'll throw a tantrum and retreat from the universe for several books, resurrecting the Band of Seven to tie you down in the meantime.
The series's heart, though, is the triangle of Inuyasha, Kagome and Kikyou. They're not the only important characters, of course. Miraku, Sango and Kohaku are woven into it inextricably too. I'd die for Sesshoumaru and Rin. However that primary triangle is what carries the series. Takahashi's playing it hard and seriously. She's conveying their pain.
1. KAGOME is a schoolgirl in modern-day Tokyo. She's normal. There's nothing special about her... until the day a topless centipede demon drags her half a millennium back into Japan's Sengoku period. (There's a lot of toplessness in the early volumes, albeit usually on corpses or grotesque demons. Takahashi doesn't do panty shots, but she seems quite keen to draw nipples.)
2. INUYASHA is a half-demon who's been dead for fifty years. He was shot by...
3. KIKYOU, a shrine maiden who died shortly after pinning Inuyasha to a tree. They were in love, but then things went badly wrong. Kagome is Kikyou's reincarnation 500 years later, which means an unusual love triangle in which Kagome is competing with (and possibly trying to kill) herself. Takahashi plays this straight, with a frightening Kikyou and everyone being very aware that no one's actually done anything wrong.
A major series motif is the fluid boundary between bad and good. Every key character crosses that line at some point, which even includes a baddie or two. Most obvious is Inuyasha, who starts out as an out-and-out villain who wants to kill everyone in his path. That never really goes away. He'll beat up children, break promises and complain about helping people. (This is funny.) Go up against him and he might cut your arm in half... lengthways. (That's in vol.5.) Furthermore, his unstable half-demon nature is liable to turn him into either a full human (with no powers) or a full demon (without humanity and capable of decapitating four humans in a single panel).
Kikyou can be terrifying. Kagome is the nearest we have to a completely good person, but the latent darkness in her soul is a plot point and of course she used to be Kikyou. More subtle, though, are the ways in which Takahashi is willing to let her be an appalling person in the early books. I sense a debt to the style of Takahashi's comedies. Kagome's awfully quick with empty apologies (although she gets better), seems to have a weakness for worrying men and is shocking in her treatment of that nice lad who wanted to go on a date with her.
Then we have the supporting cast. Miroku is a womanising thief and scoundrel (and priest). Sango's brother is... ouch. Sango is... well, there's potential for danger when it comes to her brother. All this is clearly deliberate, but Kouga's an awkward character, because with him one gets the sense that Takahashi surprised herself with a one-off villain and decided to keep him around, hoping we'd forget his origins. When we first meet them, Kouga and his fellow wolf demons kill and eat a village. They slaughter women and children. After that, though, the manga forgets about that and Kouga becomes a silly and rather annoying ally who fights baddies and keeps having shouting matches with Inuyasha. His two sidekicks in particular couldn't be more harmless or cuddly.
In fairness, this fits the manga's theme. The cast's full of bad people who learned better. That's what it's about. However at least we never forget what Inuyasha and Sesshoumaru used to be, whereas with Kouga one senses that Takahashi's trying to pretend he never ate anyone.
Oh, and while I'm nitpicking, I don't get Myouga. He's supposedly a coward who runs away from danger... but he's a flea. He's flea-sized. You only notice him if he jumps on your face and starts talking. He's never in any danger because anything that can trouble Inuyasha and co. will certainly be too big to see or care about Myouga.
Then there's Inuyasha's brother, Sesshoumaru. He's a fangirl favourite (and I can see that), but he's also a stone-cold monster who'll decapitate you for standing near him. He's a shark on legs. No one survives meeting him until Inuyasha shows up, whereupon Kagome getting angry with him should have been a Darwin Fail. I'd get scared for villains when they were fighting Sesshoumaru, even though they were evil and I wanted them dead. However... RIN.
I like this series a lot. It does have pacing issues, but I think they only become significant towards the end. Takahashi's basically continuing as always, but the dramatic spikes are so powerful that it's a little annoying when afterwards it's just business as usual again. (Takahashi can hit you hard when she wants to. She's willing to kill regulars and she can wring startling emotions from the deaths of characters you wouldn't have thought had it in them.) Besides, some of my favourite chapters are the more light-hearted ones. I love the fact that Kagome's commuting between 15th-century demon-fighting and her ordinary schoolgirl life. Inuyasha vs. modern life can be glorious.
This manga is by turns tragedy, light comedy and an imaginatively grotesque horror anthology. (Vol.16 has small children hugging their parents' severed heads alongside their half-eaten corpses.) It has a bit more fighting than I'd have chosen, but in fairness it is a shounen manga. To tell the truth, this is actually the second time I've read it.