I've been using 3.12 with 14.04 beta2(using the gnome staging ppa). I can't say it's very different from 3.10, but it feels a bit faster, and they fixed a few UX problems in the settings menus etc. Most extensions I wanted work with it. There are a few bugs that are probably due to working with an unstable distro with an unstable gnome ppa, but overall it's nice.