Love Google Now. Once you get in the habit of using it and figuring out everything it can do (which is A LOT) you'll wonder why you haven't started using it sooner.
It's funny, we have these amazing mini computers in our pockets that, (in some cases) can do SO much more than our laptops or desktops can do, yet people continue to use them the exact same way they've been using them for the past 5+ years.
Personally I don't notice too much of a battery drain if I use it sparingly. Some days I don't use it at all, but the voice activation is always on and ready for my "ok Google" command, and that doesn't affect the battery.
I use it the most when I'm driving and it's sitting in the car dock. Just like Swype or SwiftKey, the more you use it the better it gets.
It also comes in handy when showing off your Android device to your iPhone friends. [emoji41]
S5 tap'n