It's genuinely different. (Okay, there's also been stuff like Osmosis Jones, but even so this isn't something you see every day.) It's about the life of the cells in your body. It's a surprisingly lovable combination of fun characters, seriously educational content and ultra-violence.
Firstly, the scientific accuracy. There are points where the show's toned down the gruesomeness of what really happens, but that's just taking things from "insanely" to "very very very". What happens in your body every day is massive death and carnage. Your body contains 37.2 trillion cells, most of which have a lifespan in days even when something hideous doesn't happen to them. (White blood cells might live for about thirteen days, although it varies since there are lots of different kinds of them. Red blood cells for about 120 days and a liver cell for about eighteen months.)
Your initial impressions of this show will probably be:
(a) Red Blood Cell is lovely! The platelets! I want to hug them all.
(b) AMAZING GORE. The white blood cells hunt and kill bad things all day, every day... and they don't use guns. They've got knives. Big ones. (Well, except for Macrophage and her terrifying meat cleavers, mallets, baseball bats, axes, etc. Do not mess with Macrophage. You wouldn't want to, though, since she's sweet and maternal even when battering Staphylococcus bacteria into bloody chunks.) If I had to choose one image to represent this show, it would be serene, sexy Macrophage in the title sequence with her frilly maid outfit, happy smile and hand that's dripping with someone else's blood.
That said, though, the plotting's a bit one-dimensional. The cast are all hard-working cells. They have their own personalities and foibles, but they'll almost always be focused on doing their jobs. (That's even paradoxically true of the baddies, which are similarly fulfilling their function as bacteria or whatever. Cancer's a special case, though.) This restricts the plotting. Almost every episode is A Bad Real Thing That Can Happen To Your Body. The cast's personalities bring a lot of colour and humour to that, but the show, underneath, can be a bit predictable.
It's also possible that specific storylines might connect with you personally. If you have a family member with cancer, for instance, who knows how you'll react to the cancer two-parter? (I will, though, point out that those episodes were praised as "very accurate" by a cancer researcher, Dr. Satoru Otsuka, of the molecular neuro-oncology department of Emory University School of Medicine in Atlanta, Georgia.)
Our heroes include:
1. Red Blood Cell. She's a postal worker, whose job is transporting oxygen around the body. This one, alas, has no sense of direction. However she's also earnest, friendly and absolutely incapable of giving up. Listen out for her adorable little "ooo" in the show's theme song, which is mostly sung by Red Blood Cell and White Blood Cell. (In-character theme songs are the best.)
2. White Blood Cell (a neutrophil). Completely bone-white from head to toe, both in clothes and skin colour. (Except when splattered with blood, which is usually.) Stoic, humourless and devoted to his incredibly violent job, but also very fond of Red Blood Cell. Thinking about it, he's lovable in a dog-like way. Having him around is like knowing a big dangerous dog who's also your best friend and the most loyal, obedient big lunk in the world.
3. Killer T Cell (another kind of white blood cell). A loud, macho soldier. He can be funny as a supporting character, e.g. his bitch fight with NK Cell, but I don't think he works as a protagonist in ep.9. I got a bit bored by that one.
4. Macrophage (yet another kind of white blood cell). Multi-functional, both in the human body and within this show. Her sexy design is both funny and a refreshing change from the overwhelming masculinity of other white blood cells when she's saving them and slaughtering bacteria. However it's also logical and appropriate, since she's also a teacher and protector of young erythroblasts. It's accurate in other ways too. She's tall and wears a big hoop skirt, making her physically larger than other cells... which is true of real macrophages. And she's voiced by Kikuko Inoue.
5. The platelets. Good Lord, they're adorable. My only complaint is that they're not in enough episodes. (They're small children, which again is accurate since platelets really are smaller than other cells.)
The baddies are fun too. They'll all become a gushing blood fountain when White Blood Cell finds them (and he's one of the body's less scary defenders), so it's lucky that they're all outrageous monsters. (Again, with the exception of Cancer.) What's cool, though, is that their appearances are sort of accurate. Do a google image search for pneumococcus, then watch this episode. It fits. They've given it a human form and a face, but you can see the similarity. Ditto for staphylococcus aureus, streptococcus pyogenes, etc. Admittedly their version of pseudomonas aeruginosa is absurd, but it's also amusing in a comedy cartoon way.
As for viruses, they're malevolent balls with bits sticking out (again resembling real viruses), which glue on to cells' heads like evil hats and turn them into zombies. And wait until you see hay fever.
My only quibble is with the numbers. If you represented 37.2 trillion cells as people, that could populate a galaxy. What we see looks town-sized... but you could make it work if you hypothesised that the buildings, roads, etc. must be cells too. If every brick in every building in this show were a cell, that might fix the scale and also make the show a more satisfying model of what's inside your body.
Do I have a favourite episode? Maybe ep.4, for the distress and triumph of Eosinophil? Or maybe ep.6 for the hilariously macho childishness of Killer T Cell and NK Cell? Hell, almost the whole show's fun.
BRILLIANT DISCLAIMER ON THE MANGA: "these are depictions created for this comic, and differ from scientific fact." What, you mean the platelets in my blood don't really look like primary school children and I can't hope that Red Blood Cell and White Blood Cell have pink babies together?