There are very few series that give you a fresh feeling when you read it. Most manga - yeah, there are different settings and whatnot, but deconstructed, they're essentially repeats of the same old things with maybe one or two new twists.
xxxHolic is one of those few series. It takes on the Natsume Yuujinchou sort of story, but rips out 50% of that serene feeling, adds in 50% more shounen with a touch of Clamp's original comedic quirk, and takes away enough of the horror from these supernatural stories to not feel dreadful but still be enigmatic. If I had to describe it, it is Natsume Yuujinchou but sped up 2x, more heartwrenching than heartwarming, and has kickass characters.
I will give one warning. It is not action-packed, not immensely exciting. You have the other twin to the pair, Tsubasa Reservoir Chronicles, for that. To enjoy this, you have to appreciate the time & dialogue spent on character relationships. It's all about character development. And Clamp has genius character designs for xxxHolic, so it's not a bad way to go at all.
I have to say that the anime is a so-so experience in comparison because it doesn't deal with character relationships as well.
The one complaint I have about this series is that they did one hell of a job screwing up loose ends and making it dependent on reading TRC for the climax.
It was a really dumb idea to conclude Himawari's story by marrying her to a stranger. Have her pass away or marry her to Doumeki! That was not a satisfactory way to end a lead support character's part.
It was a really bad idea to include the stone, but it was a worse idea to conclude the stone by saying "it's not time to use it yet". The stone appeared to be something to make Watanuki forget everything about Yuuko. That's just counterproductive. Doumeki should've presented the stone as a choice and have Watanuki decisively make a decision on that.
I did not enjoy the fact that the central plot was disregarded. the audience had to guess most of it and read through TRC to get hints of what was happening. Not enough was explained. Everything should've just came out. Simply reading xxxHolic, people wouldn't even understand why Watanuki was starting to lose his memories. It was just a huge mess.
That being said, the denouement following
Yuko's death
was great. I loved it to bits with the exception of the screwed up loose ends.
tl;dr The main, central plot that connects xxxHolic to TRC is a huge, abominable mess, but this series is good enough to earn a good rating despite that.