Things has probably become my most used application- both on my Mac and on my iPod Touch. Okay there are still a couple of bugs- mainly with reordering to-do's on the iPhone version, and the fact that it doesn't always update to the available list immediately, but Things to me, in my work has become indispensable- even almost replacing iCal. If the developers put in Calendar facilities, then it would.

I would even go as far to say that Apple should buy Things and bundle this brilliant GTD app with iWork!

Highly Recommended.