I have been there twice. On friday the mosque is full in afternoon because on friday prayer, but on other day you can go inside the mosque and look arround ( please take off your shoes when entering the Prayer Hall) inshort i will say its a beautiful combination of chinese and islamic culture,architecture and calligraphy. Walking in those narrow streets, you will feel like you are in a 2000 years old China, and you will notice arabic scripts written on every house and door. The food there is great.