There are very few mangas I give a complete 10 out of 10 for, but if anything, xxxHOLiC excels even at that- I would have given it a 100 out of 10 if possible. It's that delightful and amazing.
xxxHolic is a supernatural story about a young teenager named Watanuki Kimihiro who is troubled by his ability to see spirits and his bloodline's inheritance of drawing them close to him. One day, when running away from the usual spirits, he enters a mysterious looking building, only to find that the spirits have vanished, and that the building is inhabited by a "witch"- Yuuko Ichihara- who can
grant wishes. Watanuki wishes to be left alone by the troublesome spirits, but in return, must work at Yuuko's shop- a job that is like no other. Soon Watanuki finds himself learning more about the supernatural world, other dimensions, and stumbles upon two characters from a different universe: Sakura and Syaoran....
In a basic short: xxxHolic is not your typical run-of-the-mill manga. Exquisite in it's detailed art and subtle storytelling, this is manga at CLAMP's finest. Beginning with a series of "events" that Yuuko narrates and reveals something poignant about human nature and humanity and then slowly delving into Watanuki's mysteries and importance of being, it features excellent character depth and development. The chapters flow smoothly, going from one event to another without you really realizing it. It is slow at first, but in a good way, as it gives space for the characters to really develop and show their true colors. Eventually, it's crossovers with it's sister manga, Tsubasa Reservoir Chronicles, lead to a main focus around 100 chapters into the manga. Yuuko's exhilarating beauty, seductive and omnipotent nature with young "apprentice" Watanuki is a delightful relationship. Also featuring is a small main cast, and a small minor cast, which allows the manga to focus exclusively on these characters with respect and care, rather than going for halfhearted attempts to cover a large cast. The best thing about xxxHolic is that it is specified for those who like thought-provoking, philosophical manga that manages to be visceral and accessible. It is that quality which allows it to retain a sense of maturity and coherence which draws the reader in, while being relatively simple in plot for a good portion of the story.
Going onto specifics, however, the most wonderful aspect about this series, lies in its balance and characters. Roughly 1/3 of the manga devotes itself to episodic stories, each ending with a certain theme or saying, which may or not maybe as delightful as the viewer would wish to hear. It is here where we get a chockfull of comedy- from Watanuki's exaggerated emotions to Yuuko's fine palette and desire for food. It is here that unusually, we get the most amount of character development. Watanuki slowly transforms from a normal, spastic teenager to a more serious, refined and curious man. Yuuko herself changes- though more subtly, and as Watanuki begins to understand himself better, so do the people around him, and thus xxxHOLiC is really more of a character-based, introspective story than its sister, TRC, which is much more plot-based.
The other 2/3 of the manga go into crossovers with the other series, which in a way is tragic, because in order to fully comprehend what's going on, you're going to inevitably have to read TRC or have a very good understanding of what's going on. That is one of the greatest weaknesses about this manga, one that has put many off from reading it, but let me say it now- don't let it get to you. If you prefer to read TRC - which has quite a long and convoluted plot - then that's totally fine, but the easiest way to understand these events is to simply read some general information about TRC (information that can be obtained by going to WIkipedia, if you wish). It's also around this period that a dramatic shift in tone takes place- a very bold move, and honestly, what makes this manga fantastic in the first place. xxxHOLiC starts off as a comedy, but ends with a much darker atmosphere. It's here where Watanuki's character development really pays off, as we understand that in all things, not just wishes, just as there is a gain for something, there is also a price. Happiness is relative to everyone, and what may be one person's happiness may be another's misfortune. xxxHOLiC explores these themes and excellently, giving quite the realistic (but still with a hopeful edge to it) answers.
Not only that, but lastly, xxxHOLiC is fantastic at giving the setting and characters life. No character feels completely flat and while some additional development could have been added, each character is different in their own way, and most importantly, the relationships between the characters are amazing. Each relationship shapes the character in a completely different way, and as Yuuko said herself- it are these ties that inevitably, make us want to wish, and make us human.
I'm probably ranting at this point at how amazing xxxHOLiC really is, but it changed my entire perspective and philosophy on life, and not many things can do that. A delight that anyone should definitely read, xxxHolic will have you most likely, hooked at first sight.