I disagree. Along with notifications, Reminders is the most important addition to iOS 5 for me personally. I use it a LOT. It's nice to have a to do list not tied to any specific date, time or timespan.
I NEVER used iCal/calender app for my reminders before, because too many of them are up in the air, I can't put everything with a due date etc.
Reminders are great, right now it's up there in the top three most used apps. I have a school list, general list, music list, and internet list (to remind me what I need to DL/update).