A few notes:
* It's not obvious at all how to get music into PrimaVista. "Evernote for sheet music" makes me think that you'll have a cute editor available. I signed up to try to find it, but it looks like it doesn't exist.
* So really it's more like "Dropbox for sheet music". That's much less of a value-add for me, but I guess searching and such could be handy.
* Even after I signed up and logged in, the index still showed large buttons for "Sign up" and "Demo".
* The Android link goes to the Play Store, but the Apple link goes to /signup.
* Using an icon of an iMac made me think there'd be a native OS X app.
* Pagination is wrong, which is a pretty serious bug: http://i.imgur.com/YJPz0xa.jpg
* From the screen shown in the above screenshot, there's no way for me to exit the modal. Neither pressing Esc nor clicking outside of it do anything. I had to resort to my browser's back button.
* The page count in the library looks very clickable (since it has the 3D appearance like a button), but in fact is not.
* I found the zooming effect jarring, but I suppose others may find it useful.
* I still don't understand how to edit the metadata for items in my library.
* I could write a separate post on everything I dislike about the upload form.
All of that said, it's easy for Show HN posts to be negative, so I wanted to congratulate you on shipping and make it clear that I think PV could be a very worthwhile product. Best of luck!
[0] - https://github.com/tsmacdonald/first-collection/blob/master/...