Bakemonogatari is based on Koyomi Araragi (Arararagi if you tend to stutter names) a part vampire, part pedophile high school senior. Ever since he was saved by a local badass surfer priest/exorcist, he's been getting involved with more and more monsters (the shows title meaning “A Monster Story”). These encounters are split into well defined arcs headlined by a girl who may or may not love him making this show maybe or maybe not a harem-y show. Now before you think of every bad cliched shounen ecchi rom-com this isn't that type of show. Each of the arcs has a dark tone dealing with issues such as abandonment, alienation, or unrequited love. Not to say this is entirely a serious dramatic show, in fact most of it is quite humourous focusing on witty dialogue and Shaft's brand of zany weirdness. Being a supernatural show each of these are represented by, well, something supernatural like a invisible crab god or monkey's paw or psycho catgirl transformation, dividing the arcs into clear animal based sections: Crab, Snail, Monkey, Snail, Cat. Unfortunately, the series level of quality story wise tends to greatly fluctuate with Crab being at the top, snake at the bottom and the other three somewhere in between. This is in part to the limited number of characters and while girls from the previous arcs don't completely disappear, it usually ends up being Araragi, Arc Girl, and one other girl playing the significant roles so if the new girl is lacking, the show suffers. Still the lows would be the best parts of many other anime series and the highs are truly amazing.
The best 2D animation I've seen. Get an HD version if you can, it'll make your eyes bleed, in a good way. I could go on about every minute awesome detail of this show's animation but people much more dedicated than me already have (like the Mistakes of Youth webcomic series's blog). Now to acquire this level of quality on Shaft's not so grand budget they tend to use random cuts to just plain colour (usually black or red) and avoid scenes with heavy movement (this is somewhat alleviated in the blu-ray/dvd release) either by splicing these coloured cuts into the scene or just having the camera not show anything and have the audio of the event play over a still frame, this is painfully obvious in the Snake arc. Some people like it for the “Shaftness”, some don't, there is no real middle ground on the issue though on the odd time the show goes into full blown Shaft-ness and has long segments of weirdness (ex. live action photos, kid's drawings) it is really something to behold. Now you while notice the “ecchi” tag added to the show, if you've seen other Shaft shows (ex. Sayonara Zetsubo Sensei) you'll what to expect but basically its meta-ecchi, beginning with opening shot of the show being an ironic, "screw the audience", slow-mo panty shot. It's never overpowering or pointless, everything is either done for weird, comedic purposes or even for plot. It's a benefit to the show and even if you don't like ecchi stuff (I would consider myself one of them) it will unlikely take away from the experience.
Each arc has its own OP with the first Crab one being the default for the few episodes (first episode of a new arc and episode 12) that lack their own unique opening. Like the arcs themselves, the quality varies and may be impacted by how much you like the girl because they're the ones singing and the song's always about themselves. I personally loved the Crab OP, liked the Snake OP whiel the rest were kinda meh but this is a major case of “your mileage may vary”; the ED and background music was also very good. The voice acting is superb all around with special recognition to the two leads, Chiwa Saito (Hitagi Senjogahara) who perfectly blends tsundere, kuudere and even yandere elements into one character and Hiroshi Kamiya (Araragi) and his eccentric straight man act with distinctive yell similar to the his other roles in Shaft shows (ex. Nozomu Itoshki from Sayonara Zetsubou Sensei)
For anyone who's seen the Despair Teacher series or Arakawa Under the Bridge, Araragi will seem quite familiar though he does bring his own brand of strange, sometimes a bit creepy, flavour. While he is usually the straight man to the arc girl's antics, the Snail arc he really shows his dark side when he feels the need to the grope and even fight the ten year old girl he's trying to help. He also shows his more human side as seen in the fight he has with his sisters over Mother's Day and the maturation he goes through as the show nears its end, though admittedly I found those parts far less interesting than the arc girl's problems and the outlandish comedy. The various girls also go through some significant change by the end of their arc but it is obvious that Senjogahara (Crab) is the “other” main character as she receives far more development and airtime than the others do, which is great because she is easily the show's best and distinctive character. From the deadly collection of stationary always kept on her person to the cold verbal jabs aimed at Araragi, she manages to embody all 3 of the -deres (Tsun, Kuu and Yan). However she does show a softer more human side though the specifics are too spoilerific to put in a review. Now this inbalance doesn't not mean the other characters are not also good, Mayoi Hachikuji (Snail) is everything you'd want in a LOLi; Suruga Kanbaru (Monkey) is far more well rounded than most anime “energetic girl” who also serves fills the shows LesYay quotient: Tsubasa Hanakawa (Cat) and Nadeko Sengoku (Snake) aren't quite as good as the other arc girls but serve their purpose well enough. As for the more side characters, Meme Oshino is the laid-back, funny, badass exorcist that puts any robe wearing priest to shame with his Hawaiian shirts. He also keeps a vampire loli in the corner, called Shinobu. She plays an important role in the last arc but for the most part she just sits in the corner being awesome. Also she likes donuts. And there's also Araragi's sisters who do the previews and Senjogahara's Dad, who were, uh, good for the 2 minutes total there were on screen.
Unless you absolutely hate Shaft's style of anime, watch this show. Its an entertaining blend of dark wacky humor, serious personal drama and the paranormal.