The one thing with me, is the ability to be responsible. Children today have so many freedoms, its not funny. I think a good age is 13. Grades are the biggest part of a child's life, having good grades, straight A's for one is one good way to award students with a cellular device.
An advantage is you can track where your child is, if he or she is not home by curfew. Of course this applies to older teens and young adults. Some preteens are track-able as well, especially if they are on bikes, skateboards or walking around in the neighborhood. Parental controls are very nice and you can make everything age appropriate, much like the ipod.
Disadvantages I see are children with cell phones care less about family events, interactions with their social groups and etc. They want to text more, send pictures and be holed up in their rooms with their phones.
While cellular devices are ransacking our world, being a proactive parent in reducing the time on the phone or services available at their ages, is a good way to control what's what.
Good luck, I know many parents are facing this new trend. Good question!