Hey, played ball as well (college) and used the bat fan. I think it helped my swing. The resistance helps you to strengthen the muscles used in your swing, and can show your weakness as well (i.e. not using your lower body and hips). I only used it as a training technique. Never use it before the plate, it won't help you and you will look like a fool.
Looks like you got some other good info here, so I am just adding my experience. Usually took swings with it every day (not exclusively, though) in the off season. Just make sure you are not losing proper swing technique and repeating bad habits when swinging it. You may struggle at first, but remember that you are strengthening your existing swing. So don't worry if you feel like a wuss. I think they sell different sizes, so you can start with the smaller one and see how it works.