I had my 4S delivered on Friday as a replacement for my 3GS & I'm really impressed so far. The girlfriend has an iPhone 4 and side by side you can tell it's loads quicker.
Siri works really well, I don't even need to speak the queens English for it to understand. We spent yesterday asking it a barrage of crazy questions and some of the answers are really funny! I think my favourite So far is telling it 'I need to hide a body' it replies by asking you if you'd prefer reservoirs, swamps, dumps or mines lol! If you ask it 'who's your daddy' it replies 'You are, can we get back to work now'. There are some really funny ones if you search on google. Seems like the programers have a good sense of humor.
In all seriousness though, I think this will change the way phones work & it's only going to get better with time. Without pressing anything I can hold the phone up to my ear and say 'remind me to buy some beer from the supermarket' it will reply with 'when do you want reminding' then I say '5pm' or 'in two hours' and that's it, job done, reminder set. If you let it know the location of your supermarket it will even automatically remind you when you get there. You can ask siri if you'll need an umbrella today & it will check the local weather and tell you if it's going to rain! Very clever!