We visited on Christmas Eve 2006 and had a wonderful experience. The Pueblo generously did not charge its usual admission fees that day, although prohibited the use of cameras.
We arrived around 1530 and were provided an unofficial tour by a resident working for tips. He provided a good deal of historical insight, although discussions about the native faith of the Taos Pueblo people were declared off-limits. He did provide some insights into the flavors of their own practice of Catholicism that stem from the original religion. The tour was limited to the outdoor areas. He directed us to a number of the "shops" that were open selling Pueblo-produced breads, crafts, and souvenirs. The most impressive offered a wide range of contemporary art and pottery from Taos Pueblo and a number of other Pueblos. Most shops offered goods of a high level of craftsmanship rather than souvenirs.
The church was open to visitors for the evening service, although could in no way accomodate even a fraction of the people who had gathered by 1700. As a note, there were eventually more than one thousand people there in my estimation (perhaps 2000), so do arrive early in the day before the nearby parking is full.
After the evening service, there was a procession featuring the Virgin Mary, native dancers, riflemen, and many residents in Pueblo dress. To light this procession, there were literally one hundred bonfires of locally gathered "greasewood" (an intensely combustible but highly smoky wood). Most of these bonfires were 1-2 m tall, but the tallest two were at least 8-10m, which were quite impressive.
The Pueblo residents were very gracious hosts and remarkably welcoming for this event.
This was a good experience, and offered some genuine insight into Pueblo traditions. Dress warmly, as you will be mostly outdoors for hours.