I'm not good with reviews, so pardon if this sounds bad, but I hope to be capable of conveying my opinion about it this way, so here we go:
xxxHolic Kei is full of likeable and unique characters, both human and not.
The animal-ey Mokona, kind foxes. Shy Zashikiwarashi and somewhat bold Amewarashi. Wise and mysterious Yuuko, stoic Doumeki and his mysterious grandfather, adorable but powerful pipe fox, cute Maru and Moro and more. I think only Baccano! had this much memorable characters.
Story was pretty loose and more of a background for the characters and their interactions, but nonetheless interesting and full of supernatural and mysterious occurences.
I'd recommend this anime to anyone. While personally I do not believe in paranormal stuff, xxxHolic is very romantic and true to it's motto(I guess you could call the most used phrase in the whole show so): "There are no coincidences in this world.", and indeed. Every word spoken, every action and detail can lead to interesting and very fascinating results.