I had never heard of kbin before the Reddit shitstorm began.

Kbin seems to be AGPL3 software targeting ActivityPub, so ultimately it doesn't matter, they're joining us rest. Seems like it's written in PHP so it could at least attract different contributors than what Lemmy would.

AFAIK, its USP is that it combines Lemmy community's and Mastodon hastags into magazines, which makes it easier to peruse certain subjects (if you're interested in microblogging). Also, the server handles a lot of the technical aspects of finding and subscribing to communities. So you can just follow a magazine about gaming instead of having to subsceibe to all the gaming communities you can find. The downsides are that you give away a lot of agency to a third party (and we know how that can end). Also, AFAIK the Kbin interface displays a lot of personal information, like your upvotes and subscriptions. It seems very oriented to following people over communities.

Kbin users do contribute to Lemmy communities and vice versa, so that is great. Personally, I feel Kbin adds a bit of bloat to the Lemmy experience, with the hashtags and complicated topic headers and all. But that is a minor annoyance.