People don't think about China as a place with very old Muslim sites, but this place is really a priceless one.
This Mosque's history goes back to the year of 742, that's just slightly over 100 years after Quran was written! Because Xi'an was an important point on the Silk Road, Muslim religion managed to reach this remote place in the very beginning of it's history.
Also, the architecture of this mosque is very unusual. It looks more like Chinese temple.
So the site definitely is worth visiting.