Immediately after I installed iOS4 on my iPhone 3G I reported that it seemed surprisingly fast. However, within two days, whatever pleasant surprise I felt about my iPhone 3G iOS performance came crashing down, and I do mean crashing.
The iPhone iBooks application, for example, crashes, a lot. Try to open a PDF in iBooks? Crash. Try to read a book? Crash. Time to wait for iBooks to open after you've downloaded 10 eBooks? I have no idea, I'm not that patient.
The Facebook app on my iPhone 3G w/ iOS 4? It crashes at least once out of every five startups.