I've been using my free account for a while now. I like the way it "thinks" about entering music, especially using the (typewriter) keyboard. The mouse entry method works well, although I prefer BiaB's method.
Coincidentally, I'll be over at a friend's house updating the lyrics on a music project I did for her. Having the notation is Noteflight made it convenient, since I can make the edits on her computer. I could probably have shared it if she had set up a Noteflight account of her own, but the point of
me working on the project was so she wouldn't have to deal with the software. (I had done the backing tracks in BiaB, and she was very happy with them).
I've had some minor irritants with Noteflight's automatic layout. It tries to be as "smart" as possible when laying things out. But when dealing with multiple competing elements (chords, lyrics, dynamics, performance instructions) it can get confused. It doesn't know about number lyrics, so the numbers don't
quite line up. I can't get it to do underlines on melissimas, although the documentation says that it's supported.
Edit: Got the melissimas figured out! The music looks much better.
But those are minor things - for the most part, it's a very impressive program.