(Why there is a 5-files limit, mods make it to 8 plz
)
As for putting it on the phone, indeed it'll double the size of your phone. However it does has advantage with that: BETTER GRIP. Since the phone is much deeper into your palm, you have a more secure grip on it. Also as others mentioned, typing on the keyboard with this thing is way easier with the case on.
Now as for the power performance? I charged it to full this afternoon and left the office the same time as I do with my stock battery where it'll be fully charged. I had a 30 minute phone call with screen on and speakerphone, among with bunch of text, PUSH and some browsing, by the time I got home 3 hours later (went to Fry's), the battery is at 90%. With my stock battery, WITHOUT the 30 min screen on and speakerphone on phone call I just mentioned, it'll be down to 85% at best, let alone some heavy use.
So far, I am loving it. More experiences and tests coming the next couple days.