Yes, many times. Hoi An is my favourite, and Sa Pa well worth a visit, but the only thing I like about Sai Gon is that our family there are still 100% Chinese, whereas in Da Nang they are 50% to 75% Vietnamese. As I love Chinese cuisine and am less keen on Vietnamese (pho bo being a notable exception), I enjoy visiting Sai Gon because I know we will eat roast duck and crispy pork there !