The current method of showing Growl-like alerts doesn't work very well. If I close it, it goes away forever. The nice thing about the Notification Center is that the alerts can stick around later, so you can easily look and see what you missed.
This has been available since Mountain Lion, and Yosemite is going to be released soon. Can you please add support for Notification Center?
Jon's suggestion is spot-on. I go back and forth between computer and off-desk tasks all day, so I miss Growl notifications. The thing I loved about Notification Center ("Reminders" app was good when all I had was simple tasks) was that the reminder stayed on desktop until I saw/closed it. Please do consider adding Notification Center support, and thanks!