We've covered this a couple of times before, but in short, the main difference between Bamboo and Intuos (aside from sensitivity) is that Bamboo doesn't do pen tilt.
Intuos can sense the angle the pen is being held at which allows it to emulate a real brush/pen/airbrush/whatever more effectively. In reality the tilt would affect the type of contact the tool would have with the drawing surface, so it gives more control and a more realistic feel with the right software. This is obviously important for professional illustrators, but if you're just going to use it for basic line work, a Bamboo would be adequate.
I have an Intuos 3. It's bloody lovely. If you can afford the extra (they are significantly more than the lower end models) it's well worth investing in one.