My experience pretty much mirrors Paddy's. Once I identify something as spam, it never comes back. Google is also very good about helping me unsub from junk. Some places where I've shopped auto add you to their mailing list, others ask. I then click on the unsub button ONCE. If that doesn't work, I mark it spam the next time and Google often says, "Would you like us to help you unsub?" I click yes and never hear from these folks again.
I will say that I have a few things that wind up as spam that aren't, and I keep marking them not spam, but they still are marked spam by Google. I know exactly why, and it irks me, but I live with it.