BusyToDo was the first app I thought of when I considered replacing Apple’s built-in reminders. I use BusyCal on my Mac, and it works great. The app still has an early-iOS aesthetic, versus the new iOS 7 look, and that resulted in a few glitches, like banners (e.g. “Due Today”) showing below the actual reminders.
The one thing that astonishes me though is that Custom Reminders (i.e. having an event repeat every other day), which are recognized by both Apple’s Reminders AND BusyToDo, are not something you can create in BusyToDo. I end up having to create my events on my iOS device and then adjust them with BusyCal on my Mac, which is frustrating. I don’t know if this is a limitation of iOS for some reason, but if not, I hope it is something addressed in a future update.
I haven’t experienced any of the crashes other users mentioned, but I would like to see sub-tasks akin to how Things manages projects.