I've tried out a few of these apps, and I eventually settled on Inkflow, which I've been pretty pleased with. My purposes are different, though; I've been using it to do initial design work for technical drawings, sketching out proportions, label positions, and stuff like that, with the final diagrams being done from scratch on the PC. If you need precision on the drawing itself, I'm not sure how useful Inkflow would be; I don't think there's a way to modify a stroke after placing it, for example, without exporting to some other program.