by Ryan Block
All our tests were conducted with 3G on, WiFi on (not connected), Bluetooth off, no data fetching enabled (unless specified otherwise). Media tested with stock headphones, medium volume, and medium screen brightness, auto-brightness disabled.
* Music (continuous playback, large library, occasionally turning on screen): 31h 23m
* Video (continuous playback, no push/fetch data): 7h 5m
* Video (continuous playback, with push and 15 minute fetch data): 5h 24m
* Daily data use (browsing, email, and GPS / maps): ~6h 30m
Those numbers definitely are not bad, but if you're anything like us and you kill time on your phone reading feeds and checking email like a fiend, by 3 or 4pm you'll likely be wondering if you'll even make it home with any power left -- especially if you leave on the 3G data. So be warned, because the kind of prolonged usage you used to get away with on the original iPhone probably isn't possible with the iPhone 3G. For some, this may be an issue. Others may never notice.
There have been a number of other fixes to better the device as well. For example, the phone now has two proximity sensors to better detect when it's held to your ear. We also found that while the camera was essentially identical, we were getting images that were ever so slightly sharper and crisper than the original iPhone on 1.1.4 (check it out below). Still, knowing that HTC's Touch Diamond -- which features a 3.2 megapixel sensor and mechanical autofocus -- could pack such a great camera in an even smaller form factor than the iPhone's left us pining for something a bit more than the same 2 megapixels from the first time around.
Gallery: iPhone 3G review - camera test
What we're probably the most excited about, though, is that two of our biggest hardware-related gripes from the original device have finally been addressed: first, the headphone jack is now flush, which means any standard (3.5mm) headphones will work in the iPhone without the need for an adapter. The new jack has a solid, confidence-inspiring feel that won't leave you worrying about damaging the device or your headphones. To this day we still have no clue why Apple pushed the jack in -- it was kind of funny hearing Steve pitch the flush jack as a feature at WWDC. It's the simple things, you know?
Second, the speaker volume has been jacked up significantly, giving your calls (or music) a much more workable volume level if you're not blessed with superhuman hearing. It's not the loudest speaker we've ever heard on a device, and unlike many Nokia Nseries phones, it's still mono. But it's definitely a step up compared to the first iPhone, which was not only quiet, but also seemed to distort at much lower volumes.

source :
http://www.engadget.com/