xxxHolic is GREAT.
The character of the story is itself unique, like Watanuki and Yuuko.
Watanuki being the clueless but skillful chara.
And Yuuko being the chara that know a lot and mysterious.
I especially like Yuuko saying that every event has fate entangled in it.
Every occurence has meaning behind it.
Besides according to me, xxxHolic is kinda the link between most of CLAMP series. and also provide some guidance to it.
And i kinda like CLAMP working both TRC and xxxHolic together, and making some connection between the manga. Must be fun!
btw, according to me, the answer to both TRC and xxxHolic will lay on the connection with Cardcaptor Sakura, what do you think?