I'm currently using the battery/back cover advertised on that ebay page second from the top (the red white and yellow one). It's rated (written on the battery) at 3.7v, 3600mAh. I'm happy with it so far. For being ludicrously cheap (red flag in my book), it gives me about 1.75x my OEM batteries run time. Clicking on "battery saver" shows the charge percentage/estimated run time. With my OEM battery fully charged, it said approx 8hrs when I set my screen to low, closed my apps (although a few posts above this it is mentioned that back-button tapping really doesn't help battery life, which I didn't know, and thus I thanked the member)...etc. And with the new (bulky, but oddly comfortable in the hand) battery, it reads 14 hours.
Now I don't expect anyone to put stock in my numbers above as literal values, since those are such arbitrary values because of how easily that estimation can change from use. But regardless of the values, they are proportional I've noticed. Whether it's 8 and 14, or if I'm using it a lot and the charges last 4 and 8-10, the bigger battery always lasts close to, sometimes more than, double the original.
The big battery adds a little bit of weight, but nothing extreme. I mentioned above it felt better in the hand, and I'm very serious about that sentiment. The original back-plate and battery makes my phone (now that I've tried the big battery) feel too thin, and it doesn't offer a way to hold the phone securely without curling my fingers over the sides, resulting in occasional button mishaps, and when the phone is warm from extensive use, my fingers sometimes activate the screen sensor which is frustrating when it thinks I meant to tap a link on a web page or something to that effect. With the larger back on the phone, it shifts the phone's center of gravity down, and the rectangular outset for the battery fits in a palm nicely. With the phone being heavier, lower, it helps it's balance and in my opinion, it seats itself very comfortably/naturally into my hand.
So after that glowing review, I'll put a rather necessary disclaimer. I just got this battery last week, so it may destroy my phone or have some sort of serious issue that I'm not yet aware of, or simply hasn't yet happened. But so far, so good. If anyone knows the specs or any actual test results or data about this battery, please post them up for reference. Also, if my phone is a time bomb and I somehow missed the review warning, I'd appreciate a heads up. hahaha.
EDIT: That listing isn't the exact battery I have. Upon reading the actual ebay page for the battery (as opposed to seeing the picture and assuming) I see that it's only a 3000mAh, and it ships out of Hong Kong. Mine was purchased on Amazon, it's marked as 3600mAh on the battery itself, and it shipped from Jacksonville FL. I know the battery is chinese, not...Floridian, but I prefer ordering even the cheapest off-brand crap from within the country to avoid the unpredictable mess associated with internationally shipping something from China. That being said, it's visually identical to the battery listed there on ebay, which leads me to believe it's the same brand. Whatever brand that may be.....gotta love the Chinese for that marketing style.