I found it actually quite hard to write exactly between the lines. I used an iPad, like you. When I upload the resulting PDF, it doesn't look great because the letters not on the same horizontal line. I tried to correct them and ran the procedure a second time, but it's going to take, in my estimation, at least an hour to get them just right. Not much for such a great tool, though.
But in general, I find it a bit hard to neatly write on the iPad. It doesn't matter much, I just use it to keep track of notes while programming and designing software. And I can read it myself just fine.
I think it's because the Pencil (version 1) feels more slippery than pen/paper while writing.
The advantage of writing on the iPad though: no more paper.