I upgraded from a 2nd gen Nano to this 4th gen Nano and I am very happy with my purchase. The screen is big and bright and the controls are very intuitive. The only two things I fault this unit for are battery life and Apple lowering the control wheel to accomodate the bigger screen. The battery life just doesn't last as long as my previous Nano. The other thing is that Apple lowered the controls to make space for the screen. This makes the unit harder to handle one handed especially if you have large hands like I do. Other then those two things, I love this device.
Say you're listening to a song you really like and want to hear other tracks that go great with it. The Genius feature finds the songs in your music library that go great together and makes a Genius playlist for you. It's like having your own highly intelligent, personal DJ.