I'm an Mbin user.
Mbin is a fork of kbin. Kbin's dev didn't really trust people much, so he wanted to have sole control over what code gets added to kbin. Which led to issues when he wasn't available and development just came to a halt for months because no one could accept changes anymore. The other devs wanted more control so they could actually get shit done, so they decided to fork the project instead.
How different is it from Lemmy? I hear they have better integration with Mastodon.
I think the biggest difference is really the fact that you can subscribe to not just communities but also users. This is where the superior Mastodon compatibility comes into play by allowing us to see posts that don't mention communities. Lemmy only sees Mastodon posts if they mention a community explicitly or an Mbin user has interacted with it.
There's also other stuff like public upvotes, boosting, tags, reputation (karma), and custom community CSS. I don't really know Lemmy well enough to give a full list of where they differ.