I volunteered with emergency first aid for a few years. Prior to that I had done a one day course on first aid but had found that I forgot it after a few weeks and would not have had the confidence to use it. Volunteering meant that I attended more rigourous training, had continuous training (every 2 weeks) and was strongly encouraged to do as many real first aid duties as possible.
It was the practical aspect of it all that allowed me to really learn the first aid. It would be very difficult to be able to do first aid without having done any practical training. When you train you have to touch the practice dummies to make sure you do everything in the right place and with the right force. The more detailed training means that you also practice other skills on other people (putting people in the recovery position, bandaging, etc). We also used make up to try to give a sense of real injuries. The guy who did the make up was brilliant as the first time I saw a 'bleeding arm injury' it was very realistic and helped me to steady my nerves before treating. To have seen a real piece of glass sticking out of a real injury the first time you have to treat it would have been extremely difficult.
It does depend on what you intend to do with it. If it's just a point of interest then a local one day course may be sufficient. The first aid at work is slightly different as there is a legal requirement to keep that up to date so you would do further training. If you want the confidence to be able to help when you see an accident then it's really up to you to decide what will give you that confidence. I didn't feel I had the confidence until I'd had some experience supported by colleagues. Since then I have approached to offer help when I was just a passer by on a few occasions, though I was still relieved when the emergency services appeared and I was able to pass responsibility over to them.
Personally I think first aid should be taught to everyone from an early age. The more familiar people can become with administering first aid then the more confidence people would have to do it and the more likely you would see people stopping to help. You see videos of people who haven't stopped to help someone on the street and lots of comments about how horrible people are. I'd like to believe that it's actually mostly a case of not knowing how to help that prevents people from stepping up. So if even the one day course enables you to have the confidence to step up to a stranger to ask if they need help and then ring the emergency services I think it's a good thing.