I have heard of this case, the only issue that I find which may pose a problem or issue (depending on the person) is that it is essentially a battery (extended) that needs to be installed into the iPhone 4 and also having a replacement backing to the iPhone to adjust to the new bulk. So if you are ok with tinkering with the backplate on the iPhone 4 it might be ok.
I cannot comment on the performance on the 2840 maH battery however and its performance. Hope this helps!