The only reason I haven't upgraded to the DSi yet, is because I don't have the money. If I did have the money (which is about $100 onto what I already have) I'd definitely buy one because A. I don't have a charger for my DS Lite and B. It has some amazing features.
But when I did have the option to buy a DSi, I didn't. Mostly because I was hoping to use the money for something else and I would miss my DS Lite, seeing as I would have to turn it in for the extra cash.