I'm not as thrilled by this app as others seem to be. For email cards from iPhone-taken photos, OK, but that's only if you like crappy iPhone photos. Email photo cards from your real-camera photos uploaded to the iPhone? Sure, but why bother to do all that (computer, iPhone, iTunes, etc.)? Why not just send the photo and message via regular email on your computer?
U.S. Postal Mail cards? I admit I'm waiting for my first one (a test) to arrive so I haven't seen them yet, but I don't see how a 960 pixel wide image will produce a high quality 8.25 inches wide print (116 pixels per inch). I'll be sure to post here when I receive the test postcard (containing a landscape image from my Canon XSi).