I've just been researching this myself.
My understanding is that the 9.99 package is for non-smartphones. It gives you limited bandwidth, and if you exceed it, then they'll charge you 20 cents per MB.
So if you don't have a smartphone (like the Droid or Blackberry) and you don't use the phone for a lot of web stuff or a lot of email, then you should be fine with the 9.99 plan.