I am kinda a fan of xxxHolic. I enjoyed both of the seasons and I think Doumeki is hot. But that is out of topic.
xxxHolic Shumuki is the OVA which was released in winter 2008 and consists of 2 episodes.
The story pretty much is basically a short continuation from the second season. Watanuki has to repay the price to those people who have saved him back then: Himawari - the promised of true smile, Doumeki - trust and gratitude and then there was Syaoran.
Then Haruka-san requested that Watanuki helped him out with 5 things since he wanted to thank him, and so together with Doumeki, he completed those tasks. At the end of the tasks, Watanuki met Sakura-chan, Syaoran's girlfriend.
Apparently Watanuki and Syaoran are alike.
Eh?!!! I just realized that Syaoran and Sakura are from Tsubasa Chronicles. Could this really be? No wonder Sakura seemed so familiar... Crossover FTW? Because Syaoran did say that Watanuki is his other self. WTF I'm confused!
Then Watanuki got back from dreamland, he met Yuuko and promised to fulfill her wish. Looks like we might have a third season coming? Yay!!
After I read Glo's comment on his top ten worst anime characters, I begin to see Watanuki in different light. I have always like him and still does I guess but I can see that he can really be annoying at times too, which was why he got a spot on Glo's list. To be fair though he was made/designed to be the comic relief of this show. There are no changes on other characters as well. Doumeki and Yuuko are still the same quirky and eccentric.
The same can be said about the voice actors and the art/animation. No changes from the two seasons which provide consistency and in my book consistency is good. The OP song was pretty chillaxing and catchy while the ED was more melodious.
If you want to watch this OVA, I suggest that you watch the two seasons first otherwise you'll be thoroughly out of the loops.