I love MP6. The regular WordPress UI just has too much shades and gradients - I like flat shades with contrasting colors very much.
One issue is that when using the Royal Slider plugin, I noticed that my media inserts are messed up. When I click the "Insert Into Post" button to insert an image into a slide, the superimposed media window goes gray and does nothing. When I deactivate MP6, I have no problem with this.
The reason this is a weird issue is because I thought that MP6 was just a bunch of CSS.