I am asking the same questions right now... my son and step son (turning 8 and 9 in Oct and Nov) have DS's that my mom bought them 2.5 years ago. They are still working and in pretty good shape, but they really want the DSi's. The local gamestop if offering a pretty good trade in deal and the kids want to upgrade... and they have the money to pay the difference from saving up money from allowances, tooth fairy, b-days etc.
I'm not sure about it due to the fact that it has internet capability.... are you able to limit that? We had a bad youtube experience recently (they were looking up something about a Mario game and somehow found animated porn... yep... Mario and the princess doing the nasty. The computer is in the living room and DH literally leviatated off the couch to shut it off, but it made for a conversation I didn't want to have quite yet). SO, that kind of turns me off about it... on the other hand I am trying to teach them to save up for stuff they want, and they have.... so what to do... what to do?!?!