I am a big fan of xxxHolic -- both the anime and the manga. I just finished reading the final volume of it in English which finally came out; I'd had it on preorder for months! I never actually read Tsubasa, though I cheated and read about it in a Clamp wiki so I could have some kind of vague understanding of its connection to xxxHolic.

I really need to read it properly, though. That should be MY next thing I read.
I can't say enough about how awesome Tsubasa is, and I've just started reading xxxHolic this week, finally. I had to pry them out of my partner's greedy clutches! I pretty much know the story, as she's been giving my the cliffnotes while she's been reading but I love CLAMP so much that I want to read it for myself.