I knew CLAMP is famous for building a huge interconnected world so I kept this on my wish list for more than 5 years but I finally gathered up the courage to take the plunge and I wasn't disappointed.
First of all, my god the art is beautiful! Yuuko's kimono, the shop, the artifacts, they are all drawn so majestically. I loved the delicate, intricate, and charismatic artwork in xxxHolic.
The characters were really interesting too. I really liked the mysterious and strong yet sometimes fragile aura that first Yuuko had then Watanuki took on. I was gradually drawn in like a firefly to a candle. I absolutely adore Doumeki and his caring for Watanuki. I liked that shounen ai-ish mood in the air~ although if you're not a fan, you might be feel put off by it.
The plot is at first, a series of mysteries with the customers coming to make wishes and Watanuki being sent to solve them. However, the real plot is gradually revealed and oh my, is it good. If you read this before TC as I did, you will definitely be confused and not understand what the heck is going on. I definitely recommend reading TC first to avoid that. I read TC right away after reading xxxHolic because I was so confused by the end. I think Clamp's style is more about showing events that occur without really explaining them so the reader has to figure out what happens. I've come to find myself enjoying this style but I have to say it's definitely frustrating and requires some patience. I liked the overall plot but I liked the day to day events with customers coming to make wishes too. It was fun to see Watanuki constantly tripping and falling as he tried to "solve" each wish. Um, I'd have to say it's sort of like Gintama style where you just have to enjoy the ride as each of the overarching plot's detail is revealed one by one.
Tl;dr - It's definitely worth the read. Art, story is excellent but read Tsubasa Reservoir Chronicle first.