Context: I've had an iPhone 3GS for about two years now. I love it, but the design is dated, it's slow, and the camera sucks. Besides your usual new gadget envy, I was excited about the fact that the 4S supposedly solves all my gripes with my last iPhone.
Design:
It's really pretty. I've been envious of friends' iPhone 4's ever since they came out, and this has the exact same body. Fit and finish is top-class, and the glass-sandwich construction pretty much never scratches or wears down like polycarbonate or aluminum. On the flip side, don't drop it.
The screen is magic. It looks like the pixels are painted onto the glass, not displayed behind it. Movies, pictures, and text look great. It's not as good for long-term reading as a Kindle, but it's smaller, and beats out the Kindle in low light (and with multimedia, obviously). I wish it was a little bigger (closer to 4"), but until iOS 5 is resolution-independent, I think there would be too many compromises to hit that size.
Performance:
It never lags, doing anything, ever. Not much to say about performance except that the machine disappears behinds its interface. You don't care about how much RAM it has or how fast the processor is, because it finishes what you ask it to do before you're ready to ask it something else. Cell reception, Wifi speed, and GPS is also way better than the 3GS- no more awkward figure-8 waving to help it get a lock.
The camera:
The images look fine, kind of like a basic point and shoot from the last few years. It won't go head-to-head with a DSLR or even a fancy compact, but in adequate light, the pictures don't look half bad.
The picture quality on the camera isn't the killer feature, though: it's the speed. I can have the phone out of my pocket and snapping a photo in about two seconds, and take 1-2 pictures a second after that. If you've ever tried to use a cameraphone before, you know how big this is.
Siri:
I talk to her like a normal person. She's good at alarms, timers, reminders, unit conversions, Algebra, and basic maps searches. If I drove more, I'd probably get more out of her, but I've had her text people for me while I'm on my bike (via a bluetooth headset) a few times.
Yeah, I could live without Siri. You could, too. But using it is fun, and magical, and why the heck not?
Conclusion:
iPhones are awesome. This is the best iPhone. It makes me smile frequently. Want a lengthier take on it? There are plenty out there.
