I've been using my iPhone 5S as my bike computer for over a year now in temperatures from -20C to +30C. I agree with the battery life comments and I use a Mophie case that doubles battery life and provides extra storage. My main issue is poor cell coverage so I lose a lot of battery life while in my office. My 25 km round trip commute takes about 20% of regular battery.
I use a delta smartphone caddy on my handlebars and have had no security issues, even when my bike and I found ourselves suddenly horizontal (I didn't fall off as I was still in full contact with the bike!). The handlebar mount comes with a waterproof bag that you can still use the touchscreen with but I have found that regular ziplock bags work just as well and I just stuff a few in my bike bag!