Just installed WordPress 2.7 RC1, I like this admin much more. Although people comment it’s to grey and solo-coloured. ( Take the CSS file and set some colors then….), it also seems more user friendly, but is a little bit slower.
Nothing broke in the currently used themes, although they warned us for that.
To say it short. It’s worth the update. Also non of my plugins broke. But I did not yet test it with Gengo.