This depends on what you might want to do.
For example, if you're developing a game and want it to run on Metro/Modern UI but also on other Windows versions, MonoGame is a great option!
It will allow you to develop for and run on:
- All Windows desktops
- Windows 8 Metro/Modern UI
- Mac OS X
- Linux
- XBOX 360
- iOS
- Android
- Windows Phone
And possibly more platforms on the future.
In my honest opinion having such a wealth of target platforms warrants any learning curve you might have to develop for MonoGame. And not to mention that you can develop in the comfort of Visual Studio, so IMHO it's something to be considered even if your app is not strictly a game (who said you can't develop a non-game with a game framework...?).
Update: Here's a tutorial, and by the way, you may want to also consider HTML5 for games or non-games. Cut the Rope (very fun game) uses this, and it seems to be doing pretty well (I didn't even notice it was HTML5!).