I think it's awesome! I love the UI, it feels simple, but at the same time more advanced than the PS3's UI. The PlayStation Store only takes like 3 seconds to load now, which is crazy compared to how long it takes to load on the PS3. The games look better than on videos that they showed (of course, since they have to compress the video). The lightbar on the DualShock 4 is really cool, I like how player 1 is blue and player 2 is red, I like that, how every player gets a color other than just a red light appearing over a number like on the DualShock 3. I know a lot of people say it shines on the TV, but no, not really. As long as the screen on the game is bright and not on a black loading screen, you will hardly see it. You can also just hold the controller down out of the screen so it won't shine. But anyway, even if does shine on your screen (which it doesn't on mine) it's so dim where you can barely notice it! The SHARE button is also a cool feature, I like taking screenshots and saving them to my Hard-Drive sometimes. I can see where you are playing a game (like Assassin's Creed IV) and you are running around, then you see this awesome view, you can just press the SHARE button and save a screenshot for forever keeping (as long as you don't delete it, remove the Hard-Drive, etc)!
If you have any questions about the PlayStation 4, ask them in the comments and I'll try to answer them!