I have to be the voice of negativity here. DH and I discussed this last year for THREE MONTHS when my 7 yo son wanted a DS from "Santa."
We got it for him (the whole, "disappoint your kids earlier or later" is a spinoff thread, LOL) and while it has been useful for me as a parent (like on airplanes!) it is a LOT OF WORK for me also. My son very easily gets obsessed by it, particularly when he has a new game.
He gets a hefty allowance and can therefore BUY a new game each month or so if he wishes (our allowance is a dollar/year each week and they buy all their own toys, games or snazzy/fancy clothes). I try to let "their money" be "their money" and not put strings on it. He often chooses to buy a game.
That means he has plenty of new games and many many times that the 30 minutes a day limit we set is pushed, and getting him off is a hassle, and it's the first thing he thinks of when he wakes up, etc. It takes effort to get him to do other things; he doesn't do them spontaneously and I see less creativity and art coming from him. Reading and activity levels have remained the same.
Part of this may be the fact the he's very into the Pokemon games and they are (I think) written to keep "pulling the child in" to the next level, the next battle, until they finally "beat the game" and things blessedly die down in our house until he buys a new Pokemon game for his DS.
I sound wishy-washy and I am. This game has been one of the harder things for me to "parent around" in my child's life. I know as they get older that the parenting gets more complex, but dang it! I didn't want a silly handheld video game to be one of those triggers. Separation, friendship, hormones, FINE, but a video game? I was kind of blind-sided by how much this has affected our family.
It goes without saying that this is completely anecdotal!