After driving 30 miles out to the grand town of SODDY DAISY to do a quick fix at a job site and get some pizza (it's damn good food, and totally worth the drive)...
I'm losing around 12% per hour of usage while running Google Now, which isn't much different than it was before. It's hard to say exactly how much different, since I never really kept track of my battery life before this little experiment. To get the most out of it, I need to repeat the last 24 hours without GN running in the background to see how it does comparatively.
But either way, the drain might be there, but it's not what I'd call significant.
edit: this isn't to say that people experiencing some extreme drain while running GN are just a bunch of fanboys imagining problems so they can gripe about Google. I keep all my running background processes trimmed down about as low as I can to save on battery life. I don't have Twitter, Facebook, or anything installed. I use my iPhone only for getting information (cuz nothing sucks more than being out in the middle of nowhere and only having 5% battery life), and talking to people I need to talk to. The fun stuff is saved for my iPad (so you all don't think I'm some kind of stone cold antisocial stoic dork or something).
Off the top of my head, only the App Store, Fahrenheit, and Mailbox give me updates at all without any direct intervention.
It's possible some people who are having problems with GN because it stacks on top of some other background process in a weird way, and causes battery drain.