The UI seems to run pretty fine so far. The drag & drop functions are superb. Only bug I ran into is that groups are not saved. To reproduce simply create or edit a group, then change a property (speed, acceleration, ...) on a robotic part. All parts will be moved to the first default group and all created groups are deleted. Same happens when going from VAB to launch.
A minor bug (that doesn't have any influence on gameplay) is the texture of the buttons 'Toggle Positive Movement' and 'Toggle Negative Movement' on the 'Servo Controls'-Panel during flight. When clicking either button the parts in the group start to move and the button's texture changes. When clicking either of the other movement buttons (eg. 'Hold to move to default position') the original toggle movement action is stopped. However the button will keep it's toggled texture.
This doesn't happen when clicking 'Stop' or the other toggle movement button, as the button's texture is reset in those cases.
The parts themself seem to work fine, as far as I tested.