I've never read Tsubasa, I didn't like it at all from what I saw of the anime. While Tsubasa needs other anime to fall back for its story because it's a crossover series (it's unoriginality kills it for me), xxxHOLiC does not, it stands on its own with very minor happenings crossing with the Tsubasa world. None of which have impacted xxxHOLiC in any way and I've read up to the very latest chapter~ ^_^

There may be a chapter or two that doesn't add up unless you're following Tsubasa (out of 117)... but hey, at least you're in the dark and going "WTH is this about?!" as much as Watanuki! xD

If you actaully wanted to read it, download it, I wouldn't spend money on it.