Late to the party but...
Batteries can charge at a maximum safe rate. Charging faster will cause heating. Most devices have components that will restrict/stop/whatever flow to provide relative safety.
The charger used should be fairly close to maximum acceptable amperage. If it isn't, then it can overheat and cause a reduced life expectancy.
Short take: you can usually use a charger with higher capacity on the typical device that manages flow. Using a underpowered charger may work but is prone to breakage and can be a fire hazard. Slowed charging cycles can reduce battery life.
If your device is rated to charge via USB, then a .5A charger should not affect battery life. The max currant for 5V via USB is 500 mA. But, that assumes the charger provides the 5V recommended and isn't reduced trying to output at is max capacity. Cheap chargers have cheap components that can fail to maintain quoted spec.
Last edited by JKenP; 10-12-2012 at 02:23 PM.