Alright, I've used that Send to Kindle page more times than I can count. From what I've seen, it's pretty straightforward but also a bit rigid. They accept MOBI, PDF, and DOC/DOCX files. EPUB was a huge deal when they finally added support a while back, which was a relief because that's what most of my sideloaded books are in. I've also sent over JPEGs and PNGs for personal documents or comics, and it works fine.
What I've noticed is that MOBI files sometimes keep your last page read and notes, while PDFs can be a pain if the formatting's off. The real trick is keeping the file size under 50MB, otherwise the upload just fails silently and you're left wondering what happened. For HTML files, you have to compress them into a ZIP first, which is an extra step I always forget. It gets the job done, but I wouldn't call it a seamless experience.