The app is faster than the website, features some slicker interfaces and is generally designed to be more pleasant to use. I've had fun building it and I hope you love using it!
Here are some reasons I think you should upgrade.
The Bullet app is fast. Apps built natively rather than through a web browser are able to utilize the maximum power of your device. It's infinitely more satisfying to navigate around when it's virtually instant.
Adding items is quicker than ever. There's a handy big '+' button at the bottom that allows you to quickly add a new entry in any context.
Swiping is the hottest mobile interaction and I've introduced it to the app in a few ways. You can swipe on screens to get to the next day, week, month or year. You can swipe to complete a task, and you can swipe to migrate.