I think the DSi is a great new system for people to get introduced to the Nintendo DS if they don't have one. It's larger screen and compact shape will do it good, and the cameras with built in editing will be fun for people who like that sort of thing.
However, at least in my opinion, the loss of the GBA slot is not worth it. For the hand held gamer there are still many gems in the GBA collection, and being able to play those on the DS is nice. Even if you have an SP for that purpose, sometimes you just want the brighter and better picture that you get for those games on the DS.
So again, for people that have never had a DS, you'll want to get the DSi. For the gamer that wants a more versatile hand held platform with greater gaming options, the DS Lite or even the original DS will be your better bet.