Impressions if you care-
Picked up a black DSi this afternoon and have only had a small amount of time to mess around with it. (Can't comment on lines and such because I was lucky enough to get a reserve ticket from an
awesome guy (

Dcharlie

), and just walked into the store, paid, and left.)
Right off the bat, the most noticeable differences for me are:
-The sound quality has been improved. The speakers are much LOUDER at the maximum setting. I ran both the same game in both DSi and the DS Lite at the same time, and I could barely hear the Lite's compared to the DSi's superior speakers.
-The screens are only a little better. It's pretty much just because they are a bit bigger on the DSi, and it helps, but it's not a huge difference to me.
-The DSi feels better in your hands than the DS Lite does. It is more comfortable to hold, and the raised L and R buttons feel MUCH better. The d-pad and face buttons don't stick out as much as the Lite's, and feel a bit "clickier" in comparison. I need to try out something like Bleach DS 2nd and see how the d-pad really fares.
-Volume BUTTONS! This is much better than the slider.
-The power button is now at the bottom left of the DSi face. You tap it once to go back to the new DSi menu screen (It has a Wii-like interface complete with background sounds), hold it to turn the unit off.
-The matte finish feels very different from the glossy, Apple-like DS Lite. I am not sure if I like this better or not over the Lite.
-The unit is a bit thinner than the DS Lite, but also a bit longer.
-The AC adapter is slightly larger than the previous model DS's, and will not work with previous models. Nor will the older DS ACs work with the DSi.
-For those who make use of certain cards for homebrew and such, they do not work.
Also a small warning to potential importers:
THERE ARE NO ENGLISH MENU OPTION SETTINGS. This is just like the Wii and further proof that the DS Stores are going to be separated by region.
Haven't messed with the other stuff like the camera yet; I'm sure it will get some laughs but it's just a novelty to me.
EDIT- Messed with the camera. Yeah, you can do some goofy stuff with it. My wife liked it for a few minutes. Yay. I am sure if I were a teenage Japanese girl I'd love it, but whatever. It's a worse camera than the one on my iPhone, lolz. Can't comment on the internet browser since you have to download it first and I need to get wi-fi working on it. Plus I don't really care.
Overall it doesn't feel like a huge leap to me like the DS Lite was over the DS; part of me wonders if they really released this to try and combat the piracy issue. But I am a sucker for new tech so I had to get one. I wouldn't recommend importing it unless you also have a Japanese Wii and buy stuff from the J-store.