I got one on Friday (UK) so I can weigh in with some impressions having put my DSi through its paces for the last couple of days.
Warioware: Snapped: A fun little game, it reminds me of the eyetoy, although I also had some difficulty with it. The camera works with contrast, so you need to have a wall which is totally different to your skin or hair colour - my flat is white/beige so I struggled to make it work, and had to resort to playing in front of a red curtain. When it works though, it's fun, and it revels in replaying some images of you looking silly at the end of the minigame rounds. Though you can't really 'lose', which makes it fun, but nothing too hardcore; no high scores or anything. A camera-in-games tech demo dressed in WarioWare clothing, but still quite fun.
DSi Key points: The DSi itself is very impressive. The pixellation is not noticeable- it really isn't, but the screen size is, it feels 'roomier', going back to a Lite immediately feels cramped in comparison. The D-Pad is flatter, 'clickier' and more responsive, and the points between it are more defined, so it won't go up instead of right, etc, ever.
The Photo and Music Tools: are a lot of fun. No, you won't be using it for a serious camera or mp3 player (although with a decent SD card, the mp3 playback is very capable), but the editing software is diverse and accomplished - more than just a throwaway extra.
The screens: Big, clear and very bright. If you do a google image search on the web browser (which is slow but decent), you'll see just how epically crisp, sharp and colourful the screens are. They're a total marvel.