A. Don't buy one of the tiny key-chain sprays. Ideally, you should buy three cans of spray. The second can is for you to carry for a year. The third can replaces the second can of spray, which you will again practice with at the end of the first year. Spray is a better option than foam for most civilian and law enforcement usage.
Streamers require greater accuracy than is usually practical.
It does not take a particularly high SHU to be effective. As with any force option, training and preparedness are vital. It may be valuable as a backup to your land line in the home - smarter criminals cut phone lines. As with many tools, the more you have and are trained to use under crisis, the better your odds. Whether your plan is to escape or fight, distance is your friend.