It's a small iPad :-)
The screen resolution is ridiculous for this size.
Upgrading from an older iPhone makes it feel like my iPhone just got really fast overnight. By that I mean that since all of my contacts and settings were automatically applied to the new phone, the final user experience is like someone upgraded my hardware, but it's still MY phone. The experience is unlike any other phone upgrade.
The camera performs really well in good lighting. The video at 720p feels like cheap video but is still of a high resolution. By that, I mean that I was expecting a more film like quality, but it is still quite good. The film like aspect can probably be coaxed out by changing the frame rate down to 24 fps in post production.
The multitasking implementation is good enough if the apps support fast switching. They don't all yet.
The front facing camera will be useful once Skype updates to use the new hardware. I haven't made a facetime call yet, but I can certainly say that the camera quality is far cower than the back camera, according to my preliminary camera tests.