I have an add on internal one in my S5 and doesn't appear to affect NFC significantly. It covers about 2/3 of the battery.
http://www.kogan.com/au/buy/qi-wireless-charging-card-galaxy-s5/

It has of late been getting warmer than I like on the charger. Also max charge rate is about 6-700mA compared with 1200-1500mA if connected via the micro USB.
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Christian
#40104192 120Gb (no longer in my E36 M3, won't fit the E46 M3)