I must agree on the comments regarding BOB as being a great dust collector for most people.
My brother (who particiapates in no martial arts training, unless you count the art of drinking liquor) has one that sits in a corner acting as a fearsome, pink rubber limbless bodyguard.
I have demanded on several occasions that he allow me to take possession of it on principle, but he won't budge.
That being said, the BOB can be a great tool for simulating a greater sense of realism in targeting and striking another person. I would recommend looking for one on ebay as the above poster did, since they can be quite expensive.
Oh, and this may be obvious, but make sure to wear gloves when hitting, since the rubber surface can tear bare hands apart.