Tried it on my build and I'm amazed by the clarity of the UI. But, like the video stated, not everything supports HiDPI. I wasn't able to test the 1280x720 resolution (mine only went up to 960x540), but I did get a taste of it, and boy it looks like something to look forward to. The amount of detail is very noticeable, and I would liken it to a 3GS - iPhone 4(S) difference. I've attached a snippet of the Display preference pane (I didn't know what else to check) and I hope you can see the difference it makes.
Edit: Just wanted to add i'm using a 23" Acer S231HL Display.