Deploymate is a simple concept, executed wonderfully by developer Ivan Vasic. Deploymate quickly and easily tells you if you are using an API in your iOS or OS X project that isn't present on the minimum OS your app is configured to support. As any experienced Cocoa and Cocoa Touch developer can tell you, missing these small availability details is incredibly easy. It's also incredibly painful when you do.
With any luck, you might catch the problem in development, but more than likely you'll ship an app or an update that breaks for thousands or millions of people. That can be incredibly costly as marketing plans are disrupted, user data is potentially lost, and bad press clogs the pipes. In many cases, you'll never get back that good will you just wasted with your customers.
At $19.99, Deploymate is an absolute steal. It would be money well spent at 5 times that price. Think about it. Is $20, $40, or even $100 worth it to you to avoid a failed launch or being at the mercy of an expedited app review? I'm pretty sure it is.