Personally, I've been decided myself whether or not I should purchase a DS at all. I'm torn, because I'm very fickle with money and luxury possessions.
But in all honestly, I do not think the DSi is worth it. The audio capabilities on it are sub-par, and it has no ability to play MP3's. The camera is nice, but almost any cell phone has it nowadays, and it's nothing special. It has upgradable firmware, and a "slow" internet browser, though free. It has a slightly bigger screen, though same resolution. It's slightly longer in length, but thinner. Storage space is the only thing I see that's really any good about it.
And in the process of all its additions, it loses its GBA functionality. I think that's the biggest blow.