For me, iPhone OS 4.0 rendered my old iPhone 3G close to unusable. Applications ran slow, sliding the red button to answer phone calls could take 2-3 seconds, typing was slow, and general use of the device felt slow. It seems I am not the only one with these issues, and Flickr user Adrian Nier recently published a movie showing the differences between an iPhone 3G running OS 3.1.3 and an iPhone 3G running OS 4.0:

If you also experience these slowdown issues with your old iPhone 3G you can downgrade it back to OS 3.1.3 while waiting for Apple to patch up OS 4 so it’s usable on the older devices. iClarified has an excellent tutorial on how to downgrade your phone which I followed to downgrade my iPhone 3G this morning. The downgrade process is very straightforward, just take note that you cannot restore a backup from iPhone OS 4.0 to OS 3.1.3 so you might need to reinstall all applications and loose all your settings and data.