Though exfoliating brushes are quite popular, and my seem like they would help clean your face more effectively (like your electric toothbrush), they can actually damage the protective layer of your skin and frequently make acne worse. Some benefit can be achieved for users with very mild acne with few, rare pimples, but it is definitely not a good idea to use with moderate or severe acne when the skin is already very sensitive and inflamed.
Some rules of thumb:
 If you have inflammatory acne, never use these brushes. They will be too irritating and damage the outer protective layer of your skin, making your acne worse
2) Choose a brush that has longer bristles (example: The Clarisonic Green acne cleansing brush heads), which will be gentler on your skin.
3) Don't press it into your face: The Brush will exfoliate your skin anyway. You do not want to overdo it!
4) Clean it thoroughly after every use: These brushes are a “perfect” reservoir for dirt and bacteria. If they are not cleaned properly they may even make your acne worse. The best way is to rinse the brush thoroughly under running water and leave it in alcohol for a few hours every week.