I've never used an official Google one, so I don't know. I like them though...they are very light and work exactly as advertised. They have a soft-grip plastic on them which I really like (keeps stuff from sliding off). I don't know what the normal charge rate is for wireless charging, but it seems comparable at least to USB levels. I can keep my Nexus 7 screen on and it will still charge despite the drain from the display.
Wireless Non Slip Qi Charger Charge Pad for Galaxy S5 S4 NOTE3 2 s IV Nexus 5 | eBay
Those are the ones I got. The packaging was haphazard and was obviously opened...but they all work and they all look new. I am probably going to buy a few for my sister as well. I heard the Note series also has wireless charging.