I'm definitely a donor - i'm uncertain if I would take one myself or not.
My younger brother was killed (at only 17 years old) in a car accident. He had opted in to be an organ donor, and my family agreed as well. He donated his heart, liver, both lungs, both kidneys and his pancreas to people who needed them, and in doing so, saved five lives. As well as the fact that he saved lives, it also helped my family (including my very young brothers and sisters) to better cope with the tragedy, knowing that at least something good came out of our loss.
I'm not sure about anywhere else, but in Australia at least, your family has the power to overrule any decision you make. You can put it on your drivers license, your will, your Medicare details, even tattoo it to your body - your family still gets right of refusal.
For this reason it's very important to talk to your family about your wishes, and ensure they know you want to donate your organs.