Sure you can use your phone but there are definite trade-offs. Bike computers like those from Stages are cheaper, smaller and more accurate (with sensors). They also have a better battery life, not to mention durability. These babies are designed to take some serious punishment.
11 products found in BIKE COMPUTERS
Ryder Innovation Tag Outfront Mount
Titan Cycle Computer T10 Wireless
RYDER COMPUTER 13 FUNCTION WIRELESS
Giant Computer Axact 6 Wired
Thinkrider Bluetooth Heart Rate Monitor
Ryder Innovation Tag Outfront Mount XCross Comp G+ GPS Kit