Is it a good replacement for Teams?

Well, I think so, but i'm probably biased given it's my company. Empirically we dogfood it successfully as a Teams/Slack replacement (and a WhatsApp/Signal replacement) while building it, and lots of others use it for that use case too: https://element.io/customers.

The biggest problems have been that the 'classic' mobile apps are stale, and are currently being replaced with Element X: https://element.io/blog/element-x-experience-the-future-of-e... - which far surpasses the Teams mobile clients in terms of UX & performance. Also, the E2EE reliability and UX on the old apps was also not great, but that's also fixed (at last) by Element X.

Will it allow my calendar to automatically attach Matrix/Element video calls to every appointment invite? Will it chime when such appointment is about to start?

This is the kind of stuff that corporate customers care about. They want systems that integrate well.

There seems to be addon for Matrix to do that. https://github.com/nordeck/matrix-meetings

> This is the kind of stuff that corporate customers care about. They want systems that integrate well.

Great. I just want a system that allows me to view a PDF file that my teacher sent me without having to click through a dozen menus and wait several minutes.

Yes. That's what you get from a well-integrated system. At work, I can open PDFs and Excel sheets and Powerpoints directly within Teams with a single click (and, one or two extra clicks to download an off-line copy).