Should use wifi when in wifi areas. I am not a heavy user but I haven't noticed that much data use. Try it out for a bit and then go to Settings > General > usage > Cellular usage There you can check how much data you have been using.
As with any app, the more that you use it and especially with the light on you will notice some battery drainange but I havent read any complaints of Siri related battery issues.
I'd say do some experimenting.