I heard that it was both mild and harsh. But first, let me explain....
If you mix it with water, it's supposed to be OK. But if you brush with it dry, then it is abrasive. I used baking soda with water for months as a toothpaste with no problems. Then being typically in a hurry in the morning, I was forgetting for a while (or rather thinking that it wasn't all that important) to mix it with some small amount of water. Then my teeth started to hurt. And they were hurting the most where I first put the brush to my teeth. The saliva will eventually make the baking soda not as harsh, that's why the area of initial application was hurting (I suppose). It felt so bad like when you put ice on your teeth, that I've avoided baking soda all together. After I stopped using the baking soda, my teeth quickly felt better in a few days.
That's what I read and experienced. Others might have different opinions about this. I think one should research this more. I know I want to read more about it, before declaring one way or another that using baking soda is good or bad. It seems that my teeth look healthier and whiter, so for that reason I'm glad I used it for at least a while.