i've never really heard of that website. I do know that you can get it on Amazon for only $6 more. 6 dollars is worth the piece of mind and trust, IMO.
I haven't heard of them either. I have purchased either directly from Seidio or Amazon. I have a 3500mAh that I received about 4 days ago. I just ordered a the 3200mAh(the battery door a bit nicer.
So I do have a 3500mAh from Seidio, and I will be glad to sell it. So far with daily use I haven't gone below 75-80%. The battery is great, I just don't need (2) 3000+mAh batteries.