Apps to check out:
Lecture Notes
Quill (vector)
Papyrus (vector)
Antipaper
MyScript Stylus
Freenote
EZPDF

I've tried them.  I like them all; they all have different features. Papyrus and Lecture Notes seem to me to be the ones most actively being developed.

I would say yes, you could replace paper as long as you're ready with you TPT up & running with the app up when you need to start writing.  I find I still use my phone (a Tmo Vibrant, no less) for post-it-type writing just because it's faster.