Bun ships with everything you need: package manager, test runner, bundler. No more installing npm, jest, or webpack separately. TypeScript and JSX are supported natively, so that's the end of ts-node. ESM and CommonJS coexist properly, which clears up plenty of the pain of installing libraries. The APIs are reachable straight from the global Bun variable, with autocomplete that comes in handy.
But what really got developers talking is the speed. Bun beats Node and Deno on requests per second and on messages per second from a socket server. Its package manager is seriously fast and works outside a Bun project too.
That said, it's still a young technology. Before using it on a big project, better to wait a few years and see how it develops. The risk of critical bugs is higher on young tech, and when you're stuck, you depend on the community or on yourself to fix it.
Official benchmark: https://github.com/oven-sh/bun/tree/main/bench