I'm a longtime PaperPile user; I was one of your initial beta testers, years ago. I discovered MetaPDF through PaperPile when I discovered that all of a sudden the PaperPile default viewer allowed annotations. I was blown away -- I've waited years for this, and has made my PaperPile experience complete. This is literally the FIRST TIME that i have been able to annotate (and read other people's annotations) on PDFs on my Chromebook, and have them saved to the original PDFs in The Right Way... I used to have to go to a Mac to use Preview, or go to a PC and use Adobe's awful paid app, to do anything non-trivial with PDFs.
That said, I also love it that MetaPDF is available as a standalone app. Since discovering it in PaperPile, I have used it to annotate things that I would not normally import into PaperPile (like conference programs), and to read annotations on things that I not ready to import into PaperPile yet (like reviewer feedback on manuscripts I have submitted).
So, basically, thank you for MetaPDF! I see that shapes, text boxes, form-filling, signatures, and page management are on your development list. I applaud this: at that point MetaPDF will be able to everything that MacOS Preview (and the paid Adobe app) can do, but on the web and for free... though I would probably be willing to pay. If you're collecting data about user priorities, I would use form-filling the most often, followed by page management (though there are several free-but-clunky web apps out there for merging and splitting PDFs). Shapes and text boxes are lower priority for me, as are signatures... and there are some decent (paid) apps out there that do online PDF signatures.