one good thing about it is i don't accidentally cover up the speaker grill anymore, but damn that thing is so bulky, it makes people around me look at me like i'm using the nokia 5110 phone.I've been using a mophie juice pack for about 2 weeks. I bought it because I'm about to take a trip and I'll be away from my hotel for 20 hours at a time.
So far I am digging it. I gave it a couple real world tests, using a lot of music and internet, etc. It does definitely add some bulk to the phone, but I don't mind. I think the phone is easier to grip now, it's easier to hold to my shoulder with my ear, and I even gave it a good drop the other day and it protected the phone excellently.
An added bonus is the way the mophie redirects the iPhone speaker towards you (instead of just down) while you're not using headphones. I also like that you can sync/charge without removing the phone via the usb cable. It charged the phones battery, as well as the juice pack simultaneously.
As stated earlier in the thread, it will give you between a 70% to 80% charge if you're just about drained. I am a fairly heavy user and with the mophie I can go about 2 full days without needing to charge. I'm rarely in a situation where I can't charge overnight, but it's nice to know I have some backup, especially since I'm headed out of town next week.
There are some other competitors out there, but I recommend the mophie if you don't mind adding a little thickness to your phone.
I found some refurbs on eBay (cosmetic stuff) for half the price, the one I got has just the tiniest little scratch that no one would ever notice if they weren't hunting for it.
My one complaint about the mophie is that it seems to attract an insane amount of dust around the camera.