Checkmark made the rounds a couple of weeks ago when it was announced. I have been beta testing the app for a good while now and can say that it is a really solid app. I don’t personally use it, but that has more to do with my love affair with OmniFocus than Checkmark itself.
Shawn Blanc uses the app, and after talking with him, I agree that Checkmark is best thought of as a Reminders Pro — that is Apple’s own Reminders app taken to the next level.
The best explanation I can give as to why you should use Checkmark: if you love location based reminders, but you don’t love having to enter them by hand in Reminders. Checkmark really excels at this and has an added nicety of delaying the reminder when you arrive or leave a place — that’s actually exceedingly helpful.
It’s $0.99 in the App Store and really worth checking out.