I have two of the 5200mah batteries that look like the ones in the OP. I have been very happy with them, and battery profiling in battery monitor widget pro seemed to indicate they were definitely delivering around the 5000mah range. I'm generally impressed how much life i get out of them. I bought the first one as part of a back+charger kit from bestdigitalusa on ebay, and the second one was from gadgettown on ebay.
It is very possible there are counterfeits out there, though. I have bought li-ion batteries from china before and had them break open in shipping and revealed that only half the battery had li-ion cells in it, the other half was double sided tape and metal weights... I would not be surprised at all if some of these extended batteries turned out to be cheap 2000mah cells with some weights and spacers inside a 5200mah package. Fortunately the ones I got do not appear to be that.
The included charger and back are quite convenient, but I would recommend anyone using them regularly expect to have to buy new ones at least once a year since the back wears out and the charger pins are flimsy and break easily. But for $5 that isn't so bad. I love to be able to charge batteries while I'm out and never have plug in my phone or be tethered to a charger, especially while traveling. Every time I get home I can quickly switch batteries and suddenly be 100% again.