To me the Nintendo DSi is a cheap attempt from Nintendo to a new generation of handheld by adding a slightly more powerful CPU, SD-card support, removal of the GBA slot and a fancy low-res camera.
Maybe somebody who never owned a Nintendo DS in the past might find it interesting, but to me (who already own a Nintendo DS Lite) it is too expensive to be worth of an upgrade.
Also there are some nice homebrews on the Nintendo DS that are very interesting but they obviously require a flashcart.
If you don't mind about GBA games and never owned a Nintendo DS in the past, I'd say it could be a better choice than a Nintendo DS Lite. It's all about personal preferences after all.