It's amazing. I even went camping on top of the mountain where there was no power source around for a dozen kilometers, but I was still able to use my mobile phone. Of course in order for me to do that I had to learn a few tricks first to reduce the iPhone 3G battery consumption and prolong the time it takes to completely drain out and to need a recharge. First of all you should change all the necessary settings in order to limit the battery consumption, that is turn the Wi-Fi off and also turn off the positioning, all the email and message alerts and the automatic run for updates.
You may also dim the display luminosity and turn off all the other features when you are not using them - for example internet connectivity that eats a lot of your battery energy. If you do all these, you will have the surprise of using your iPhone for a long time without the need of a recharge any time soon. But if you want to improve the lifespan of your iPhone 3G battery, you should take care of it, that is never depositing your device in very warm places like the glove compartment of your car because if it is in a hot environment and even in the direct action of the sun, it will seriously shorten the life time. Then consider charging the iPhone without any case because this, too, can cause the iPhone to overheat. And again - do not keep the iPhone connected to your computer unless it is on and the device is charging because if the computer is on sleep, the battery will die out.
The features of an iPhone battery are pretty good and also the results of the tests. For example it is guaranteed for up to eight hours of talk time, 250 hours standby time, six hours of internet use, seven hours of video playback and 24 hours audio playback. Any way, the only downside to this battery is the fact that you can't change it yourself after purchasing one from the internet. First of all if you do open your iPhone 3G with the intention of replacing a dead battery, your warranty will cease to cover for any damage.
Then it's pretty hard to do this because all the pieces are so small and fragile that you could break them if you are not an experienced technician. But of course the guys who are pretty good in this field and have a dead battery iPhone 3G that has just come out of warranty will risk it and open it up. If you want to do this yourself make sure you know what you are doing. Or maybe wait for the much advertised iPhone 4G to come out o the market and get this new model instead of your old 3G.
One last thing - I am very curious if any of you have tried to open an iPhone 3G and if yes how difficult was it?