Everything's been said a hundred times over in here.
Great manga, could even be a masterpiece, if it wasn't for one of the most typical and well hidden trapdoors: the middle of the manga is a bit too slow (which is when all the costumers come to the shop's owner Ichihara Yuuko).
Well, turns and events in the story make it all up to you. The character developement is some of the best you'll ever find in mangas.
General things people should know when reading xxxHolic:
Reading the Tsubasa Reservoir Chronicles (TRC) first would drastically increase the ability to understand the embedded details. These two manga are closely linked and even facets of the same story. Treat them as a series, that tell a single story, except this one isn't prequel and sequel, but rather it's all happening at the same time, actions in each of them explain certain other actions in the other.
TRC however is the easier read (mostly) and does not require xxxHolic for understanding, so it might be best to read TRC first, then xxxHolic.
I personally would refer anybody above age 18 to this story. Anybody below that age may require a re-read or two later in their lives to understand the whole manga in its entirety, all social aspects included. I am 21 as I write this comment, I've a good if not especially intricate understanding of Asian cultures and life has equipped me with a deep understanding of human psyche and yet I am sure, if I were to read this story again in five years, I'd catch on things I didn't quite see this time around.
After the introduction and the first few chapters things slow down a bit. Maybe too much, but that is personal preference and might have been neccessary to keep xxxHolic and TRC in synch, both publishing and character developement wise. Should you find that you dislike xxxHolic because of that, don't be discouraged. The real interesting stuff starts right after TRC's plot ends in xxxHolic.
The end feels kinda hurried though, incomplete. That's why it might not be very easy to identify with, even though it is quite emotional. The ending is not the worst I've read, in fact it is still one of the better ones, but it does not quite do the rest of the manga justice. Therefore I reduce the score from a 9.5 to a 9 of 10.
Note: There is a sequel planned: xxxHolic: Rei, so that might be the reason for the loose ends. That doesn't change anything for this manga by itself though, so the score stays.