xxxHolic is one of the best mangas I have ever read.
The art is beautiful, as expected from the renown CLAMP.
The story line is complicated and mysterious. You really need to follow closely otherwise you would be lost on the essential details. This story is definitely unique. I have never read anything like it. There are some fluff, but every one of them is funny, which is a good diversion (-ish) from the actual plot line. You can't really guess the actual plot line until later in the manga because there aren't enough hints in the early part of the manga. So, with that, I would say that this story is moving pretty slow, but the long read is worthed at the end.
Character development is good. I think we actually see a significant change in the character only when the actual plot line starts to show, which is maybe around the early 100 chapters or so.
For those people who say that you have to read Tsubasa manga in order to understand what's happening in xxxHolic, I must disagree to a certain extent. I read Tsubasa some times ago, but I stopped along the way for some reason I don't remember. Anyway, I remember only a rough plot of Tsubasa and am able to read xxxHolic without getting too confused. So, my response to "have to read Tsubasa" is that--just go to wikipedia and get the rough plot line because that's all you need to know.
I want to say something about the manga vs anime for those people who have watched the anime and wanted to start on the manga. Manga is obviously better than the anime (true in majority of cases). Anime follows the same plot line as the manga. There's not enough of a plot line in the anime, though, so if you want to know what happened next and in more detail, read the manga. (I apologize if I am getting any facts wrong; I watched most of the first season.)
I am commenting on this manga based on reading 163 chapters.
It is getting so much more interesting.
This is definitely one of those mangas that by killing off a good character leads to an even greater development in another character--and the story as a whole--killing off this character is needed to further the plot.
Strongly recommended for those people who love the supernatural and mystery. Warning, though, there's no serious romance in this.