iPhone 6 in back pocket, Wahoo RFLKT on the handlebars, iPhone running Cyclemeter. The RFLKT shows me all the stats I need (speed, cadence, HR, time, distance on the main page, others at a click), and the phone loads my ride to Strava the minute I finish. In this configuration GPS drains my iPhone 6 about 5-7% of battery per hour (used to be 10% but in iOS9.1 Apple have been really pushing on battery life improvements), i.e. less than if I was sitting on my butt in my office where I'd be using my phone periodically to check Facebook and the like.
The only reason I'd consider replacing this with a dedicated GPS is if I decided I really needed mapping. Right now I have to pull my phone out of my pocket to check a map if I'm lost or unsure where to turn. As 99% of my rides occur on roads I know, this rarely happens.