I'm using Simplenote as platform to save notes. It has iOS app for iPhone and iPad, OSX app, web app. nvAlt also can sync with Simplenote via provided API. And it's free. Tagging, review history are also supported.

One thing I can't be satisfied with Simplenote it's iPhone app. It's very simple and UI is great but overall experience when editing and copy/pasting text is poor compared to native iOS notes.

As of this app - do you provide some API to get notes back from your app? iCloud doesn't and it's sometimes not very useful.