Hey over there,
I've tested Mugen, Seidio, Acase, qCell, and many, many more on the original Galaxy S (Epic for Sprint,) the Fascinate (Galaxy S for Verizon,) Epic Touch 4g (Galaxy S2 for Sprint,) and have been toying around with the S3 for Sprint and some friends' phones for about a month.
I'd say you have really two choices... drop the big money and buy ONE Mugen or Seidio, or just buy a couple qCell batteries. Of the five/six qCells I have used and/or tested on my friends' phones, I'd have to say that only one varied in charge capacity from 10-20% of the advertised charge capacity (4200mAH)... and that was after two months of usage that was inconsistent.
For $20 and standard Amazon Prime shipping, I'd just buy two qCells and call it a day. Even after a month of daily or every-other-day full charging, I have not noticed a "memory effect," (yes, Lithium Ion batteries have a memory effect and anybody who tells you otherwise can't spell "hysteresis").
Why buy two? First off, you're going to NEED TWO if you do deep fieldwork in urban environments like I do. There are some nights where I'm sleeping in a gutter in the middle of no where and have to have my phone on 4G to upload ethnographic observations. There are some days I will *not* have access to an electrical outlet, have to use 4G, data transfer, TeleNav, Google Maps, video capture software, sketching software, and a bunch of other *&#^ at the same time, and if you think for a SECOND that having two standard batteries is going to be enough, then you're insane. You need at *least* one, 4000mAH battery with a modern smart phone if you're going to be using these services HARD ALL DAY, and easily two after you've broken the batteries in for a few months.
(Ever try running from a crackhead in the South end of Chicago while changing your battery since you need to dial your primary investigator and tell him that you may not make it back to publish your thesis due to a massive stab wound? Yeah, that's what I thought).
Also, as a word of caution... remember... a lithium ion battery is like a horse's stomach:
1. It needs to be filled often.
2. It needs to be emptied often.
3. If it's used hard (aka extremely warm,) you need to give it a little break (let it cool down ~10 degrees F) before filling it again.
4. When you get a new one, always remember to establish a consistent feeding pattern for a while, (aka always completely empty and completely fill a new battery 10-20 times before you start charging it while half full or while the phone is on).