I recevied 2 of these this week. I ordered them about 3 weeks ago. I've been using the one for a couple days now and I have to say I am pretty happy with the results. It is lasting longer than my stock battery. How much, I can't exactly say because I tend to not kill a charge in one day and I charge every night.
Where I seemed to notice a boost was when running programs on the phone. This battery doesn't seem to be draining as fast as the stock battery. When running programs with the stock battery, it dropped pretty fast. This one seems a bit slower. Now, I have noticed(as someone else said), when it gets down to around %30 or so, it seems to lose it's charge a bit faster than when above it. So for those with the battery, keep an eye on it, when it gets low.
I'm only on my 2nd charge with these batteries, so we'll see how they progress. All in all tho, these were so worth(assuming they don't explode
) the $9.50 per battery I spent on them(paid $20 for 2, which INCLUDED shipping and got a $1 refund shortly after ordering). Plus, I got a micro USB reader with it too.