xxxHOLiC can be split into 3-4 parts. First, xxxHOLiC reads like a supernatural mystery-of-the-week. These sections were fairly interesting and I know I loved certain specific chapters, but for supernatural mini-stories tangentially connected by the protagonist I'd rather read something like Nightmare Inspector which has a more serious tone than xxxHOLiC.
Then the series skips to a very emotionally charged exploration of the main characters, making them feel more human and tightening their relationships. This to me was the pinnacle of the series because, before this, there really was no reason to care about these characters before because they were so flat and they couldn't be called much more than stereotypes; spastic weirdo, silent manly hero guy, love interest, sexy boss lady. They developed sympathetic characters traits and I found myself suddenly caring whether or not one of the main cast got hurt or not.
The next 'part' of the series is actually an ongoing theme that the series includes, and it grinds the story to a screeching halt. It is one of the things about this series that the majority of the fans hate, and with good reason. The series tries to increasingly tie itself to Tsubasa, and it is very forced and it provides ....... little to nothing of value to the universe of xxxHOLiC. It's during these sequences that you have to force yourself to read these very long-winded boring sequences of Yuuko explaining to us how the two stories are 'connected.' Except... it's not all that connected. All of this can be easily removed and none of is actually has is direct or important influence on the main story of xxxHOLiC.
As for the home stretch of the series, it makes a jarring return to the mystery-of-the-week format. It's obvious that the author started to see this series as a burden. There are a lot of pointless time-skips, and its the most jarring with the major ten-year time skip. Some main characters are forgotten and don't even show up for the last chapters. One character in particular gets so lazily written-out of the series that it was shocking. And, in the end the series goes out with a whimper. No fanfare, no emotional intensity, just a lot of mundane talking in a very repetitious sequence.
CLAMP's long-limbed characters are often the subject of much ridicule but I have to say that some of the designs, especially Yuuko's outfits and the chapter title cards, are lavish and quite pretty. The style of all the butterfly imagery to the simple flowing of smoke from a pipe is great eye-candy. That being said, panels can be lazily drawn like giving two-page spreads to pages full of black ink and/or a simplistic character portrait (especially near the end.)
I do recommend this manga... but only the middle portion of it. Don't bother reading the ending, and (only if you have time to kill) read the beginning.
