Story:
The majority of the chapters of xxxHOLiC are contained in their own story, but all contribute to the reader's understanding of the characters and what may be unfolding around them. While sometimes this can bring down the story, overall it doesn't harm it as even things that appear to have no influence on the important events do help develop the characters in outstanding ways. Occasionally it may feel as if the short stories do drag the entire thing on too long, however, but the pace improves in the later chapters and saves itself from becoming too dull (though the short stories are, for the most part, rather interesting).
It is worth stating, however, that the segments where TRC plays an important role feel out-of-place and shoehorned. They really shouldn't have been in this manga, or at least have been made to actually work with it instead of being forced in such a way.
*Note: xxxHOLiC's events and a few of its characters have very close relations to other works by Clamp, most notably Tsubasa Reservoir Chronicles, but one does not need to read these to understand the story. However, they are likely to give one a much better understanding of the universe surrounding this manga, so I recommend it.
Characters:
There is no long list of characters to consider in this, as xxxHOLiC focuses entirely on the plights of the four main characters. While they are quite one-dimensional in the beginning, they gradually get developed and grow, just as they should. The relationships especially get fleshed out and complicated ties are established between each character. By the end, no character is left undeveloped.
Despite this, I do have my grievances. The most prominent one is one other main character, seperate from the main four that consistently appear throughout the story, who appears in the later chapters. She does get to be quite important, but is phased out nearly right after she's established as a prominent character. She becomes a mere blip on the radar, and that is an issue that's never dealt with.
Art:
The art for xxxHOLiC is unconventional and stunning, in my opinion. Even the panelwork is great, and creates a wonderful balance page to page. While it may look odd in some scenes, mostly due to the appearance of the characters, its actually rather easy to get used to and becomes a pleasure to look at. I wouldn't compare it to artwork from other manga due to just how unconventional it is, but it shows great improvement from Clamp's absurdly proportioned characters in the early days.
xxxHOLiC is commonly said to be Clamp's masterpiece, or at least one of them, and I agree. It was a memorable ride, and a beauty to behold. Definitely worth reading despite its length.
