I have just tested some Micro USB Power Suplies and found out that the
Samsung S2 Suply gives good 4,9x Voltage with an whireless Keyboard Receiver
and USB Stick attached to pi even only rated with 0,7 Amps so I guess gives more and cheap at amazon.
I have others telling about 1,2 A but allready dropping to 4,6x Volts with same setup.