I got mine from Amazon also and am very pleased with the unit. Battery life for me is typically 9-10 hours, running GPS, HR and cadence. You can increase this by getting an external battery pack that plugs into the mini-USB port and then the 305 will run longer than I can pedal. The one I got is:
Tekkeon myPower go (MP1500) from geomangear.com. $15. The box holds 4 AA batteries, Nicad rechargeables and will also charge your cell phone with the right adapter. When the Garmin battery runs out, I put the Tekkeon in the seat pouch, run the cable along the top tube and plug into the Garmin. I haven't bothered to make a handlebar mount.