I'm a long-standing fan of Papers and when I saw Manuscript, I was more than happy to give it a try. These are my initial impressions after an afternoon of playing around with Manuscript.
First off, I open the app and select one of the handy templates for a journal in my discipline. It crashed Manuscript. Hm, ok. I try a different one. Also crashes. After trying 4 or 5 I give up and try a blank document. I start typing and enjoy the overall look and simplicity of the design. I decide to paste some text from a PDF file. It crashes. I try to copy a figure into the document. It crashes. I try to click between various sections, apparently too quickly for Manuscripts. It crashes. I try to insert a function. It crashes.
I have to say, I am sorely disappointed. I am really delighted by the idea of the app, and I would jump at the opportunity to replace MS Word as my regular word editor, but this app is simply not ready. Now, I realise many of the issues I mention are documented elsewhere in this forum, but I simply wanted to give a general impression of why I won't be using Manuscripts. I simply don't have the time to be faffing around with a half-finished product.
I want to like this app, but it's simply not ready for a full release, let alone charging for it. I do want to see it succeed though, so I hope this first iteration goes well and maybe I'll check it around v2.0.