The basic gist of the app, released this week, is that you can record a video, up to 12 seconds, and send it off to one of your friends. Your friend then gets a push notification and can watch it.
Unfortunately, in order to take advantage of the video recording part of this app, you have to have an iPhone 3GS. Those with the 3G or original iPhone can take a photo and record 12 seconds of audio as an alternative. The app does not require a 12seconds account, but rather allows you to link with Twitter or Facebook. It then pulls down your contacts from either service and you can video message to your heart's content.
12mail is a great concept as a private video messaging app, but it seems to have missed the mark when it came to Facebook Connect implementation. When you send a video message to one of your Twitter contacts, it shows up as a direct message on Twitter and as a private message in the iPhone app. In contrast, when you message a Facebook contact, it posts it in public on that person's Facebook Wall and shows up in the iPhone app. The result of sending a video message in these two scenarios is very different. It would serve 12mail well to alert a user on Facebook by sending an Inbox message. In its current state, the Facebook integration just doesn't work as you would expect it to.
Despite some of these functionality issues (which can be worked out), I really like the idea of private video messaging or "video voicemail." It has a lot of great potential uses and opens up a whole new way of communicating.