Love 'em. The best there are for sauces in my opinion. They have a sweeter flavor than regular plums. I have always found that it is easier for me to add acid (balsamic, lemon) than try to neutralize it at the end. There is nothing I hate more than adding sugar or honey to a good pot of marinara. I have even heard of using baking soda to neutralize the acid in tomatoes.
San Marzanos also have more flesh and when cooked down turn into more of a suace than a tomatoey water, if that makes sense.