Ok, I originally had the InCase Power Slider Case/Battery and although it had a bit of bulk (not really noticeable to me), it worked marvelously. Aside from the finicky USB sync'ng capability (where you don't need to remove the iPhone from the case), I blame that on my computer's USB problem. However, aside from being able to charge it and not have to worry about removing the iPhone from the case, it was rock solid. It would deplete the case then it would then hit the iPhone and deplete that. Simple. Now that I purchased the iPhone 3GS, I decided to try something different. I purchased the Mophie and the FastMac IV. Mophie for the simple fact that it was very similar to the InCase Power Slider due to the fact that it was also a case (2 in 1 so to speak). Also, it was slimmer then the InCase Power Slider. In addition, one of the good things also was it had a "standby" switch where you have control of which battery to go through first and/or use the case like an extended battery whenever you need it. However, I have had nothing but problems. I initially charged my iPhone to 100%, Charged the Mophie Juice Air to 100%, I depleted the iPhone down to 25% and decided to charge my iPhone (while still in the case via USB), and the iPhone won't charge? First problem....tech support said it was a cable issue. Fine... With the Mophie Juice Air still 100%, I then removed the Mophie Juice Air from Standby to initiate charging of the iPhone (as advertised by Mophie Juice Air) and literally after 4.5 hours of Mophie pumping power into the iPhone, my iPhone 3GS stopped charging at 75% and the Mophie is depleted! As advertised it's supposed to go through two rounds of charging the iPhone to 100% if you start charging the iPhone when it's at 10%! So per tech support, they are stating the device is defective!! WHAT?? Anyway, we'll see the next pass around. Now with the Fastmac IV, I'm testing it now but I do have a question with this. Could I leave the FastMac on the entire time? I'm going to say the answer is yes but it seems like the FastMac is a true extended battery where it really is not a case and although you can leave it on, it doesn't really work like the other two cases/batter (Mophie and Incase Slider). From my understanding when your iPhone is depleted the FastMac detects that it needs to be charged and then you cradle the iPhone into the Fastmac and it starts charging. Is this correct? I mean, I rather and deal with leaving it but wanted to know if this is the case. Another thing is when the iPhone is in the Fastmac, I have to always press the power button in the back if I want to charge my Iphone as well? Someone please shed some light with this. Thanks!