I haven't had the chance to use it myself, but I am interested in watching its development.

The AT protocol supposedly addresses some big issues with Mastodon and other ActivityPub-based services (like Lemmy). Notably, account portability and distributed identity. From the AT FAQ:

Account portability is the major reason why we chose to build a separate protocol. We consider portability to be crucial because it protects users from sudden bans, server shutdowns, and policy disagreements. Our solution for portability requires both signed data repositories and DIDs, neither of which are easy to retrofit into ActivityPub. The migration tools for ActivityPub are comparatively limited; they require the original server to provide a redirect and cannot migrate the user's previous data.

Having a distributed, portable identity system with built-in public key exchange is a big deal. IMO that is the single biggest problem with ActivityPub. Users should be in control of their own identity.

I follow some people on Mastodon who did the half-baked profile migration, and it's really bad from a UX perspective. Occasionally I want to find their older posts and it's difficult, certainly not search-friendly.