Is the iPhone 2g good or not?

What are the differences from the iPhone 3g?

There are some substantial differences between the original iPhone (2G) and the iPhone 3G. An objective comparison can help you decide which is best for you:
http://www.everyipod.com/iphone-faq/differences-between-the-original-iphone-and-iphone-3g.html

However, also keep in mind that the original iPhone will not be able to run iOS 4 at all and the iPhone 3G will not fully support the operating system either:
http://www.everyipod.com/iphone-faq/iphone-os-4-new-features-supported-iphone-ipod-touch-ipad-price.html

The 3G has a faster web connection when not using Wi-Fi. Using the internet on the iPhone 2G when not on Wi-Fi is similar to 56k speeds. The new iPhone 4 is coming out! If your looking for a new one get that, or get teh 3GS because its faster than the 3G and its pretty cheap now. It will also be able to upgrade to the new iOS4 operating system.