I've used Notes Plus, Upad, neu.Notes, Penultimate and Note Taker HD (NTHD).
NTHD is the best by far, imo.I use it extensively every day to prepare notes that are projected onto a classroom whiteboard. I also use it on the fly, instead of a board and marker. I really appreciate it's ability to add pics, shapes and text as well as handwritten notes.
NTHD gets more and more intuitive with each regular update; the last few updates have been real corkers. All this ease of use is due to in no small part to the experience of its author, Dan Bricklin, who wrote the first spreadsheet program, Visicalc back in the seventies.
Yes, I guess there is a learning curve, but the help documentation and videos are excellent.
I agree that Upad has got very nice ink, but NTHD's is just about as good as Upad's, especially since the last update. And features-wise, NTHD makes Upad look like a toy.