you might want to check the voltages of the original charger and the kindle charger - even if its the same fitting, different voltages provided by the charger can do harm to the device (over time)
No surprise, the Kindle port is a standard micro usb. By definition, it is set up for 5V DC. The only difference would be the current (mA) supplied to the device. Probably no big deal, either your device will charge a little faster or a little slower than the OEM charger.
FYI, I haven't even taken my OEM charger out of the box, I charge on the computer usb port normally, and in the rare instance I use a wall charger, I use a charger from my wife's old LG Dare.
So, the reply above worried me a bit. You don't think swapping out the chargers and using the droid one on the kindle (which I found) and the kindle one on the droid will be a problem?
I like keeping one each at home and work
no problem using charger for different phones.. all chargers almost have the same exact voltages and outputs.. etc.. .but you should try to use smaller text next time.. =)
idk ive heard of people using a different charger and a few months later it kills the battery. might not be everyone (or just crappy made chargers, idk)
ive also noticed (maybe its just mine) when i have my phone plugged into the usb to charge (not even mounted) it seems to get a lot hotter than when just charging using the wall charger