Warm is normal on mine. When I charge off a 2 amp usb charger it gets warm, when I charge off a lower powered usb port it does not. The 2amp charger gets the iStick charged in just over an hour and the usb port gets the job done in a little over 4 (as expected based on capacity).
Following standard battery safety rules, the iStick battery should be able to safely charge at 2 amps, and the heat produced during charging is very similar to what I would expect with other batteries. Since it has a built in charge controller, I would expect that it does not matter what the rating of the "charger" is as the internal processor will keep the charging process withing safe limits.