My dc have just turned 6 and have just got a DSi. DD will get one when she turns 6 next year. There were several reasons we waited. Personally I don't think a DS is any worse than TV, I think all screen time is equal and they all have their uses and plus points but can only speak for our Mac,DS,laptop,TV as we only have those.
We waited so we could spend the money on lovely toys(you can get a looooot of Playmobil for the price of a DS).
We also wanted our twins to look after them properly and be able to read so they could get the most out of them.
We have a 1 hour screen time rule most days with a very strict 1 hour rule on the DS. If either of the boys kick off when it's time to put them away they loose it for a week,it works a treat. We have no screen time before school.
Dtwin1 is a Playmobil/Lego addict,he's at school all day so feel he deserves down time. He still plays with his toys just as much. Dtwin 2 is an avid reader,he'll often choose reading over his DS /screen time. He's working his way through the Secret Seven books(his choice) so his Ds hasn't seen a lot of action lately.
I really like some of the games,my boys have gained a lot from Cooking Mama,Nintendogs,Brain Trainer. We just got the Drawn to Life one for Xmas.
I think all kids mature at different rates so you can't really say what is the best age however I do think it's crucial to regulate usage properly which it's perfectly possible to do. The parents who say you can't are those frightened of their own dc imho.
My boys have gained masses from TV,using the Mac, Pc and their Dsi's. They can differentiate between a Mac and Pc and do the most wizzy stuff(last night Dtwin 2 made a Magic Treehouse Reward Chart himself by finding clipart online himself and putting it into a grid he made). They are just getting to grips with the non game stuff on the DSi,there is masses you can do and I feel all of our screens really benefit all 3 of my dc.They rarely "zombify" and to be frank having spent all day at school and with homework to complete I wouldn't really be upset if they did for an hour.