This is a great deal on Amazon, two 1800 mAh batteries plus a charger for 20 bucks right now. I recommend it.
I just bought these a few days ago and got to use them yesterday. I was amazed... 300 mAh more than my year-old stock battery and it lasted forever. On my normal battery I get maybe 9 or 10 hours of light usage, data off all day unless I'm using it for tapatalk, etc. and then the phone dies. On this new one I got it up to 14 hours and it was down to 40%.. only put it on the charger because I had to sleep.
And that was 14 hours of medium to heavy usage... I had two batteries with me so I really wanted to stress them and see how long they would last. I left data and sync on all day, streamed music for about an hour with google music, used google talk to chat with friends, played fruit ninja, watched youtube videos, downloaded Big Gigantic's new album over 3G... etc and still the battery lasted way longer than my stock one. I didn't even have to worry about needing to switch batteries which was what I bought them for, I didn't expect this great of a result from it.
All in all, I recommend these if anyone is looking at buying extended batteries. It's a great price right now, the things charge in the phone and come with a wall charger with USB ports and a battery charger that can charge whichever one your phone isn't. I've never been more happy with the battery life on my phone, I don't think you could go wrong with these.