Combining Reminders with Siri on the iPhone 4S is pretty awesome. Simply ask Siri to, "Remind me to do 'X' at 'Y'," with X being the thing you need to do, and Y being when you want the reminder to go off. In Mountain Lion, Apple brings the Reminders app to the desktop, minus Siri. You get everything you've got on the iPhone, plus a couple of cool additions.
On the desktop, Reminders lets you create a task or to-do list, set the date and time you want the reminders to go off, and push them to all of your iDevices. Having Reminders on your Mac also means you have the ability to search through your reminders and view them on your calendar.
The one iOS 6 feature that Reminders doesn't do on your Mac is "Remind me to do 'X' when I get to 'Z'," with Z being the location where you want to do your task. Geofencing, as it's called, makes little sense on desktop and notebook computers, which generally don't have GPS or real location services.