I think part of it depends on the demographic of your "good customer". If you deal mainly with larger businesses it may be hard to reach them with a mailing. Most of us in the print business are primarily business-to-business, not business-to-consumer, so saturating the residents in an area with a mailer (like EDDM) usually doesn't make sense for us. If you can focus in on your "ideal customer" with a targeted mailpiece, you may see some good results. But even then you'll want to follow up with repeated mailings over the next few months. Unless you're really giving away a loss-leader, you typically won't see huge results from your first mailing.
Also keep in mind that while the postcards are cheap, the postage adds up fast. And a 4x6 card isn't going to get much attention as a marketing piece. We typically try to steer customers toward a 6 x 11 (the largest card you can mail at letter rates) or even 9 x 12 if they're willing to spend a little extra on postage. Keep in mind that if you're mailing over 200 pieces, a mailhouse can mail under their bulk mail permit. Even after you pay them for their work, they will still save you money on postage.
Our business is more print than signs, and we try to do a monthly mailing to all larger businesses within a certain radius of our location. Usually about 1800 cards per month. We try to feature a low-priced special on something that's more of a commodity item (color copies, 1-color t-shirts, 2x6 banner, etc). These deals tend to get customers in the door, and then it's up to us to add value, build a relationship, and continue selling based on service and solutions rather than price. But in our model we have found postcards to be a good marketing tool. FWIW, we send a 6 x 11 card.