I'm a usage hog. On iphone or android I'm dead by 10am-noon. So one huge perk with Android is having fully charged batteries ready to go. So since I jumped to android 3 phones ago I've always bought the extras. So yes for my S3 I have 2 of the battery/cradle combo kits made by Samsung. I start every day with 3 full batteries and never have to worry about charger cords.
I will say when I'm not working which is where my hog usage comes from, I can get 22-26 hours on a single full S3 battery.