The most common criticism is that the phone is slow after an upgrade. There are also many complaints that the phone drains the battery quickly and becomes excessively hot. Concern about general problems such as slowness and battery life on gadgets isnt unusual, but using iOS 4 on the iPhone 3G seems to make the problems so bad that the phone is nearly unusable for some people.
We've posted on these issues before. As you can see in the video below they can be quite troublesome for users affected. Hopefully, a fix will be found soon.