I'll fight back on the question - I think you're better off with a cycle computer that can sync to Strava, Garmin, or other app. First off, they mount better to your handlebars. They're built for that, where as any smartphone is by comparison large and requires an unwieldy (and possibly expensive) mount. Second, bike computers use less power, unless you don't actually use the phone while riding (i.e., you let the screen go dark) - if you're wanting to track your info (speed, HR, cadence, etc) in real time, you'll need the screen on at all times.

Now, if all you're looking to do is record your ride for reviewing at home, a smartphone can do that (I haven't used it but Cyclemeter looks very good), then you can ditch the bike computer. But I spend a lot more time looking at my ride info while riding than once I'm home.