I've not used it myself (either as an author or as a reader), but I've heard a fair bit about it from other authors. It does fairly decently for romance authors (especially erotica), but not as well for other genres. Right now, Kindle Vella is still basically in beta and only available in the U.S. and Amazon has done very little to actually make readers aware of it. They're starting to, but not like they should. They also have authors annoyed because if readers use free coins to unlock paid chapters the author gets paid nothing for them. So, looks like there are still issues to be worked out if they really want the platform to do well.