Kimihiro Watanuki is a orphaned teenaged boy who can see spirits, so his life is already complicated even before he starts working for shopkeeper and "space-time" witch Yuuko Ichihara. The elegant Yuuko, a devotee of strong drink, gourmet food, and flimsy, revealing outfits, always has plenty of arcane tasks for her young employee, along with the more mundane drudgery of keeping her shop clean and her plate and glass full.
This series starts out as a string of mostly unrelated anecdotes involving the odd situations Watanuki encounters as he works for Yuuko, but about halfway through these 8 volumes (all that are available thus far), a single, stronger plot emerges, joining Watanuki's fate to that of his classmate Domeki, a handsome strong-and-silent guy who's the descendent of powerful priests. As Watanuki and Domeki are forced to work together, both of them learn a number of truths about what it is that people (of all kinds) owe each other.
Although I enjoyed this series from the start, it didn't touch my heart until the arc about the boys' shared existence got going. However, all the way through, I appreciated the elegant, Art Deco-style artwork (although I do think its nature only increased the emotional distance I felt from the story) and the many notes that the publisher, Del Rey, has included about Japanese culture and folklore.
I find myself wondering whether Himawari, Watanuki's ever-oblivious crush, is ever going to be more than a cipher. At the moment, she seems to exist only to emphasize Watanuki's distance from normal teenagerhood and to allow the reader to hear Watanuki's views on what's been happening to him, because he tells her everything. She's amazingly unfazed by everything her tells her, so maybe she'll turn out to be more interesting than she seems thus far.
I'm also curious to know more about the twin boy and girl moppets that help Yuuko a bit in the shop and act as a preternaturally annoying Greek chorus to Yuuko's less-flattering observations about Watanuki. At one point, Watanuki generously offers to take them out to a night festival, and they sadly demur - they can't leave the shop (IIRC, it's something about their not having souls).
On the other hand, the Tsubasa crossover stuff left me pretty cold - it was delivered in a way that felt like smug name-dropping to me, and my reaction was always along the lines of "Yeah, you have another series going - get over it, and get back to the real story."
One more thing is worth noting: I had bought the first three volumes of this just to see whether I liked it when I had my Mother's Day manga spree. The Mr. has ignored most of what I bought that day, but when we were hanging out in a bookstore a couple of weeks later, he showed up at our table in the store's cafe with the remaining five volumes of xxxHolic, saying "We don't have these, do we?" So I'd say that if you're looking for a shoujo series that could appeal to guys as well, you might try this one!
I do have one big question though: how do people say the name of this series? I've been saying "ex-ex-ex-holic," but that's pretty awkward!