A few other reviewers mentioned feeling mislead about the physical dimensions of the battery, but it's almost exactly what I imagined, and the size was a concession I was willing to make to gain the extra juice before making the purchase.

The battery slides right into the battery compartment of my AT&T Galaxy S2 (i777), but the battery cover does not.  It gets infuriatingly close -- I can actually snap every mount-point into place -- but the topside of the back cover pops off without constant pressure.  Once the top pops, the entire cover pops off.  Without the cover, the battery isn't secure.  I haven't actually tried to use it yet, but I picture having the battery pop out of the phone while it's in my pocket.

It's a few cents of plastic, but without it, you can't get value out of what you purchase...

I shouldn't have to do it, but I'm going to look for a phone case that supports the larger battery.  That MIGHT be able to provide enough constant pressure to keep the battery cover in place.  Unfortunately, even if that works, it's going to make an already thick phone (with the 3,500mAh battery) even thinker...
A few other reviewers mentioned feeling mislead about the physical dimensions of the battery, but it's almost exactly what I imagined, and the size was a concession I was willing to make to gain the extra juice before making the purchase. The battery slides right into the battery compartment of my AT&T Galaxy S2 (i777), but the battery cover does not. It gets infuriatingly close -- I can actually snap every mount-point into place -- but the topside of the back cover pops off without constant pressure. Once the top pops, the entire cover pops off. Without the cover, the battery isn't secure. I haven't actually tried to use it yet, but I picture having the battery pop out of the phone while it's in my pocket. It's a few cents of plastic, but without it, you can't get value out of what you purchase... I shouldn't have to do it, but I'm going to look for a phone case that supports the larger battery. That MIGHT be able to provide enough constant pressure to keep the battery cover in place. Unfortunately, even if that works, it's going to make an already thick phone (with the 3,500mAh battery) even thinker...