Study Bibles are mostly useful for entry level Christians. The closer you get to a professional education, the less helpful they are, but since 99% of people never come close to that, they're good for 99% of people.

The one caveat is, you have to draw the distinction between the Bible and the interpretation of the note. Some people confuse them, not on purpose, but just because it's all right there in front of their face, so they take everything at face value. Notes can be mistaken, they can also single out one interpretation of a verse, when in reality, the interpretation is debated, or in some cases, there is a multi layered interpretation.  

 I probably do consider myself entry level:)