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.
XXXHolic (pronounced as 'Holic') is 24 episodes of a positively peculiar comedy, supernatural anime set in the typical Japanese high-school setting. A unique story lies in wait for those who decide to watch it, episodic as it contains multiple self-contained story arcs. A slight bit of research and this anime is made by the same people who made other anime like Cardcaptor Sakura, Chobits and Tsubasa Chronicle...