I used vistaprint for my save-the-date magnets. I was really happy with them and received some positive feedback on them. They're light and thin enough to mail without having to pay extra postage (in Canada, at least). I made some of the business-card sized magnets and some of the oversized.
The only comment I have is that they might have a bit of magnet-junk stuck to the edges that you have to clean off before you send them out. I think it happens when they cut them.