picked two 4GB up yesterday at the apple store on Regent Street. Drove into London on a whim. They got them in 20mins before I got there, and there was already a queue.
VERY thin. Still quite tall, but its the thinness thats the killer. Makes the normal ipod look huge in comparison. I like my 40GB photo, but you do have to compromise on size. With the nano, its really got to the size where its irrelevant - your headphones will probably use up more space than this thing.
The UI is pretty much identical to a photo, but smaller. Same nice click feedback on the wheel etc. The wheel is quite narrow compared to the photo, and I found myself 'falling off' when spinning a lot. Plus the main surface is quite 'sticky', so that can combine to feel a little clunky.
White looks great - black is OK but I'm not convinced by it. Maybe I'm too indoctrinated by ipods ID to accept anything other than white.
Haven't done anything with them yet - I'll probably sell the black one as I'm not fond of the look, and keep the white. I'll sell my 40GB to fund it - its *way* too big for me in comparison, and I'm pretty sure the music I actually listen to is a lot less than 4GB.
^
| Thats two stacked together BTW