A good tip could be just about anything, from a nice cone to really jagged. One problem was any of the methods one has to view the tip can't actually resolve the single atom that is doing the imaging.
Dumb question .. I get that the needle scans a surface and you get the quantum tunneling effect between the atom you are "looking at" and the tip of the needle. What I don't get is how one figures out depth. For each X,Y position, do you just keep going down until you touch something, and then move up, and go to the next position? If so, apart from the issue with 1 atom tip, I imagine the next problem would how to increment X and Y by 1 atom.
P.S. I think some of the marketing put out on these things really confuses the issue. Sure .. it gets people excited about science but it gives people the wrong intuition. As a non-physics person, I got a lot out of this article.
This means that you're getting a "pseudo-height" map - if you had a surface with 2 types of atoms, both the same size, but with different tunneling barriers, you would see them appear to be different sizes.