It's even quicker to make butter if you own a Thermomix. I've done it a few times - definitely not as many as I would have liked but I'm going to make a concious effort to make it more. Here's a video for Thermie butter:
Now we could go further into this and talk about using organic cream to make your own butter which would I'm sure everybody would do if a) they could afford it and b) they could find it but since this blog is about making small steps then just making the switch from margarine to butter is fantastic and if you want to go one step further and make your own butter that's great too. I just use normal cream I can't recall ever having seen organic cream in the supermarket so I'll have a look for it next time. One other thing to check for when buying butter is that no yellow colouring (Annatto 160b) has been added.
I generally try to go by the rule of eating Real Food. Margarine is a science experiment whereas butter is made of real food. I used to dislike the taste of butter growing up and I have only just made the change recently and my taste has changed after using it for a little while and now I like it. It's such an easy change to make but one that makes a big difference.