I have a black DSi. Here's the breakdown.
Pros:
-D-Pad and buttons (including shoulder buttons) are better than the DS Lite, but not by too much.
-Speakers are MUCH better; they are very clear and loud.
-Brighter screen options available.
-SD card storage available (although there's no real use now other than storing pictures).
-Downloadable games available through the DSi store. With Japanese units, they even give you 1000 points free.
Cons:
-Matte finish, not glossy; mine tends to have white scratch marks all over it when I take it out of the case. They wipe right off, but it's annoying.
-No GBA port.
-The screen size is only slightly bigger than the DS Lite, and it's still the same resolution.
The DSi doesn't feel like any kind of real upgrade over the DS Lite to me. If you're over the age of 10, you're not going to care about the cameras, playing with the mic, and all that nonsense, although I see a Wario Ware game is coming that will make use of the camera, so maybe some fun can be salvaged from it. My feeling is that they really made this to try and combat software pirates, but they've already failed in that regard. I wouldn't bother getting one unless you really care about the downloadable games. It's hard to believe that Nintendo is also going to be making DSi-only software. Good job splitting up your userbase!