But too hard to start using. It took me 10-15mins to discover how to enter a note: you have to click where the cursor is. And not just where the cursor is, but slightly to the right... I had to read the help, and it wasn't even the first thing!
Entering a note is the very first thing a user wants to do. For user adoption (which is the what you need when starting), you have to make that first thing incredibly easy: click somewhere, get a note. All the additional sophistication should be additional to that. (IMO)
I think they are aiming at existing users of existing tools. But I think that's too hard - they can't beat desktop applications on their own turf. There's a lot of sophistication in the tool - so it's like they are aiming at what the professional applications do. These tools have competed and evolved to meet the needs of professional composers. However, I think a web tool is really in a different market, with a different proposition. They should first focus on where they strong. It's Writely vs. Word.
Music layout is hard, and their's gets messy when it's more complex (looking at Moonlight Sonata).
The fact that you can't publish to the web, or share (except with people who are already members) makes it harder to get started. Maybe not allow the whole score to be viewed without an account, but a tempter, to entice get people to sign up for the rest. As it is, there's little evidence that it's worth signing up. But maybe focusing on dedicated users (not casual) will work? As I said, it's pretty niche.