I use my iPhone but I can't stand the Strava app and use Cyclemeter instead. I find it to be much more programmable with settings and options that Strava doesn't even come close to having. Using Cyclemeter, I can do a century and still have over 60% of my battery life at the end of the ride. I also like the fact that it displays the route map on the main screen and moving from screen to screen (11 of them) is just a swipe of the finger and not a menu or button choice. It can also display all the information that Garmin and Strava display on their web site. It will also send me and others an email at the end of my ride that contains my stats and a link to web site. If you have an iPhone 4s or 5, it has real time tracking so that anyone you have set up can follow you on your ride on another device or PC, which I don't think that Strava can do. And there's much more ......

As for the accuracy of the GPS unit in a phone vs a Garmin, I can tell you that there is a lot of misconception in this area. I did a lot of GPS based software development for an engineering company that contracted to FEMA. The software we developed had to be extremely accurate for what we were doing. The primary device was a Symbol handheld computer connected to external GPS units via BlueTooth. We tried several external GPS units and found that the accuracy between them was miniscule and that included the Garmin external receiver that we ended up using. Where the difference that you actually notice comes into play is: 1) the number and location of satellites whose signal the device is receiving, 2) the way the development software converts the binary data sent by the satellites, 3) the way the programmer parses the converted data sent from the GPS receiver, 4) the way the device's CPU handles the floating point numbers parsed by the programmer. BTW, the only reason we ended up using the Garmin receivers is because they were the only ones with a magnetic base that could be attached on a metal plate mounted to a hardhat. The larger the GPS receive, the more accurate it is (and more expensive). For what you get in a Garmin cyclometer vs a smart phone, you're not going to see large differences, if any.