Not maintained, do not use this branch on new installations. Use 5.x-2.x instead.
3.x!? Are we already at v3?
Yes, it breaks the old API in favor of a newer, better, faster API. It also strips out a lot of cruft that built up. Mostly it separates the API and UI (a la Views) so if you just need the API, you can have it, it's yours, enjoy.