I use it very occasionally to go to Zhuhai. I see very little cars on the bridge when I'm on there, so in the same section about 2-3 cars. Do pass by some buses though, since they are limited to a lower speed limit. Compared to taking the boat it's a much easier proposition for me to even just take the bus. But yeah this bridge is largely a symbolic umbilical cord right now. Doesn't make sense to me since Hong Kong is connected physically to Shenzhen.
Bridge is massively expensive but pretty useless. It always was going to be that way though IMO. Not sure why people are surprised. But you know, lots of expats got paid real well to make this happen.