This is a VERY good series. Funny, well-drawn, mysterious, and with likable characters (Watanuki is one of my favorite manga characters of all time, and Yuuko is up there, too). Near the beginning it doesn't have much of a plot (it consists more of stand-alone stories), but it gets one further on down the road.
This is a CLAMP series (they also did Cardcaptor Sakura, Chobits, X, Tokyo Babylon, Tsubasa...I could go on), but I'd recommend it even to people who normally don't like CLAMP - it's a bit unusual for a CLAMP series, IMHO.
Xxxholic also crosses over with Tsubasa: Reservoir Chronicle in places, but it stands pretty well on its own...you don't have to read both series to get this one, though it's more enjoyable if you do.