Doug Sauerhaft wrote a post about postcard marketing vs email marketing. I  don't have any direct experiences with postcard marketing, but I concurr on the email marketing. There are so many different ways to market via email, but I do think that the only true way to do it successfully (w/out pissing off people) is to market to people who have opted in. There's all kinds of ways to build an opt-in email list. But, it's certainly not going to be as timely as buying a direct mail list and sending a postcard.