The DSi is very underwhelming. In fact, on balance, I think it seems like a downgrade more than anything else, at a higher price to boot.
On the positive side, you get:
+ Slightly larger screens
+ Camera (but a very crappy one)
+ Some kind of vague wifi improvements
On the negative side:
- No GBA compatibility (this is a real bummer -- I've got lots of fun GBA games, and my GBA hardware is not doing so well)
- REGION-LOCKING
The size difference seems so incredibly slight as to be meaningless.
The region-locking is the biggest stupidity, and an ill harbinger for the future of Nintendo handhelds, but I'm guessing it (and maybe other changes to lock down the DS) is the real reason Nintendo is releasing the DSi in the first place.
The DS is such a nice little system, I guess it was naive to think Nintendo could keep up their winning streak... I'm kinda glad I bought a DS lite before the DSi becomes the only choice!