I've had iOS 5 for months now, and I initially rejected Reminders after a day or so, largely because the cool location-based feature drained my battery instantly, but also because it was too rudimentary. I don't mind simple to-do lists (I like Google's task list, and it's pretty bare bones), but I just didn't see a real use for Reminders.
With iCloud and, particularly, Siri now on the table, along with battery fixes with the location feature, I am all over Reminders. Apple has inexplicably lagged behind even Palm with having a syncing to-do system, but now with iCloud I have a list of tasks along the side of my calendar on iCal that sync with Reminders on my phone. Thanks, Apple…that's really all I wanted to begin with. I don't need tags or anything too fancy. (Apple couldn't make me too happy, though, because they took tasks OUT of Mail.app with Lion. Et tu, Apple? Why do this? You can drag an e-mail onto iCal to schedule it for a date, but I can't find any way to link mail to Reminders. Annoying.)
Back to Reminders. It's at base a checklist with some extra options: you can make your Reminders repeat and occur at certain times, and even that is fancier than Google Tasks. The cool part is you can set them to remind you about something when you arrive at or leave somewhere. This has useful use-cases and also cases where you might as well save yourself the effort. Creating a Reminder to take something with you when you leave home, for example, is useless because it won't tell you until you're a block away. I find them very helpful for when I GET somewhere, though. I drove past the library yesterday and my phone reminded me to pick up a book. That's actually very useful.
Siri's integration with Reminders makes me feel like I'm living in the future. I have a list in Reminders called Shopping List. I can say "Add dog food to my shopping list," and Siri puts it on there. Rebuke me at will for my laziness, but I'm far more likely to add stuff to lists when I don't have to unlock my phone, pull up the app, scroll to the list, and type it in. Then when I'm at the store, I can say "what's on my shopping list?" and Siri pulls the list up. Nothing about that but cool.
I use this integration a lot when I'm driving. Things generally occur to me when I shouldn't be writing or typing, and committing them to memory is dicey. Now I can say "Add blog on Reminders to my ideas list," and here I am blogging on Reminders because wouldn't you know, my phone did what I said.
Now I need an app to inspire me to actually carry out all the productivity I have scheduled.