it drains the battery with locations services constantly running which is an essential process for the app to work.
the only way to slow down the rapid depletion was to turn off location services for all the other apps. but even then when running Google Now, it drains the battery faster than any other app I know off.
also the Google Cards, doesn't work like its counterpart on the Nexus phone which pushes a notification card/alert at specific locations. in iOS the app has to be running for cards to generate & even then it still needs a tap on the refresh button.
I can forsee Apple integrating its own version of this within iOS soon. I don't use it, but from what I read, it sounds very useful, but seems pointless on iOS if the "Cards" don't pop up and as I only use search from Google I'd need to change my email, calander etc to make use of it.
Strikes a little of Google making it available in the App to say they have and its available on iOS. The fact its practically pointless being overlooked a little. Had a play, turned it off as location services was on permanently and then actually deleted the app as i realised it was on my phone and had barely been used if at all.
Strikes me as the kind of thing that will be integrated to iOS somewhere down the line.