iPhone 3GS, upgrade when flawless. Well let me say, I did a restore and then setup the phone like new. I didn't want to take any changes of having some issues pop up. I think most of the issues I've seen reported are directly related to upgrading. I say this because after they do a restore it works just fine.
I have used it a lot since the install and I notice no slow downs or lagging. I went to the gym, had Pandora playing, did some email, made a few notes on my to do list, replied to some email messages, checked stocks, even made a hotel reservation using Safari. It is so nice being able to fast switch between apps and have them come back right where you left them.
I am going to enjoy iOS4.