Don't forget the OS tie ins that truly make it a personal assistance.
I was using iOS 5.0 betas for months and found no need to use Reminders. Too many steps to creat a reminder, but with Siri it was easy to say remind me at x-time or remind me when I arrive/leave x-location.
I don't know what it was like in the Beta but in the gold master iOS5, Reminders is like, virtually the best and easiest To-Do list thing that comes with iOS5, and rivals many apps designed to do what Reminders does at that level of simplicity.... It's indispensable for me now. Notes and Alarms was a wrong way of doing it.
Of course, Siri makes it all much easier but I can't seem to find the right phrase for telling it to make a non-timed Reminder.
Edit: Found it:
Sometimes, you might want to use iOS 5′s new Reminders app as more of a checklist, rather than being reminded at a certain time for each item. Using the normal Siri query, Remind me to…, you’ll be asked to give a time or place to be reminded. In order to add a ‘dateless’ reminder, you can say one of two things. Either use Add … to my … list, which will add the item to the named list, if you have one set up, or you can use Remember to… to add the reminder to your default list without giving it a date.
And iCloud seems to be syncing Reminders quite nicely to iCal on Mac (thank gawd no more Notes in Mail), iPad etc.
Bottom line, Reminders is good, with Siri, superb.