First of all, the design of the iphone 3G could be distinguished from the first version of the iphone by the colour of the new phone. The whole back cover of the iphone is actually black in colour where else in the original iphone it was silver and black.
Secondly, with the support of 3G and version 2 firmware, the menu seems to be much faster. I do like the accelerometer rotate which is really nice. GUI is best in its class. itunes is good and boy, I love the fact that it allows third party application software install. That is good. Finally apple has realised the potential of the iphone 3G. It should support third party application. That is why people like to use Windows Mobile and also Nokia phones (running on symbian).
Finally, I think the new iphone is worth the buy but wait... If you can wait a little longer, you would be offered sim free and also pay as you go! Just have patience!!!