The Reminders list on iOS is actually pretty good. I'd been using Wunderlist but the service was really unstable (multiple extended downtimes) and their site wasn't even secured by HTTPS! Unacceptable.
The easiest way to import tasks into the Reminders App is through iCal on your Mac. Unfortunately, as far as I can tell, getting tasks from one to the other isn't exactly automated unless you use an AppleScript (which I've put together and provided below) — but, it does the job. However, once you get setup in Reminders with your data, it's pushed seamlessly between your devices and it feels great!
Apple's Mac OS X Mountain Lion (10.8, coming this summer) will have a built-in Reminders App much like the iPhone and iPad. This is what I'm personally holding out for. In the meantime, managing tasks via the iPhone, iPad, and also iCal on the Mac is a more than acceptable solution compared to what I was dealing with before.
To get started, I'm assuming you've already created the Reminders lists you want to import tasks into on your iPhone or iPad, and enabled Reminders sync to iCloud on your device(s) and also Calendar sync on your Mac through the iCloud panel in System Preferences. If this is the case, you should see the same Reminders lists on each screen (iPhone/iPad/Mac) when enabling the Reminders column in iCal under the “View” menu. (as mentioned, Mountain Lion will have its own Reminders App)
Now, there is one final preparation for getting your tasks into Reminders: Getting the data out of your existing tasks App! — This may be tricky. However, most Apps offers some method of making this happen. For example, Wunderlist enables you to email an entire task list to someone, so I simply did this but instead of sending it I just copied it into a plain-text file using TextEdit. The key is that it must be a plain text file (you can make it one using the option within the “Format” menu and you must have only 1 task per line with no skipped lines! If you fail to do either, you'll end up with additional blank or jibberish tasks in your list and that is not helpful!
Also, one fine caveat with this script is that it does not seem to support Unicode characters. This means that anything too outside of the ordinary alphanumeric characterset (such as special punctuation, or Japanese/Chinese/Korean/etc. characters) will not render properly and you'll have to manually fix them. Keep this in mind, but you're probably 99% OK as long as you use mostly standard English words and punctuation.
Here is the AppleScript. Open the AppleScript editor within your Utilities folder if you're not familiar!
Run this script while modifying each of the paths and task list names as necessary, and the data will be added to the appropriate lists in iCal and automatically synced to iCloud and your devices if setup properly.