I've been using it over my iphone 4 since dish offered the sling adapter ($99 and plugs into my vip-722 dvr) back in Nov. 2010 and it works pretty well over 3g. Seems to take a bit of time to start the streaming process, but once it figures out the speed/bandwidth of your connection it works great. This is not an issue however, when I'm on wi-fi. And like the other poster said, there is a lag between stopping/fast fwd/reverse/etc. As for amount of data used, I've never checked because I still have the unlimited plan.
Just received my wi-fi only iPad2 last nite 🙂, so I'm anxious to try it out on that.