If I can unstick them when they are stuck, that's great. If they get stuck every fifth line of code, or on simple problems, that's terrible.

Couldn't have said it better myself. I give leeway to the new hires to learn the domain-specific stuff, but it becomes quickly obvious if they aren't picking up on the material. I had a guy not understand that "4883-381" would not be interpreted by Python as a string without the quotes after 8+ months of working with the language. At one point I caught him trying to put arguments outside of parenthesis. I realized he had basically been copying and pasting the entire time, banging his head against the wall until something worked. I almost went postal.