I currently use Siri, and I noticed a slight drop in overall battery performance. I don't think it is any more that of any other app. If it is something that you think you will use I wouldn't worry about the decrease in battery life too much. I find myself using it more and more and it is great. It is the biggest selling point of the 4S anyway. I would definitly try it out, and if it is something that you either don't like or don't see using enough, then you can always turn it back off. Hopefully Apple will fix the iOS 5 battery issues soon and it wont really matter. Hope this helps.