I miss one thing most on my iMP-350, a LCD display on the player itself apart from the one on the remote. Looks like this is not included in the new version too, probably to keep it slim. But what the heck, its a great mp3 player ;-) Many people may want to have a car-kit included with the player too.
if it were me, I would buy the imp-350..... and just buy the new remote off their site..... because it looks as if the unit itself hasn't changed very much...... and it would cost under $150 as opposed to $200 for the imp-400.