Just received mine last night. Impressions are as follows:
Screen is beautiful. Very crisp, great color saturation and the size is great. I am not a fan of the huge screens but I can still reach the whole screen with my thumb and the extra space is a plus. Will be better when all apps are updated to take advantage of the extra screen space.
Speed of the phone is immense and I came from a 4S which was no slouch. Everything just feels snappier and the camera speed (without flash) is mind blowing. It will take pictures as fast as you can press the button lol.
Weight. It is noticeably lighter but I don't find it mind blowing. It is also very thin but again not mind blowing.
Design. Beautiful. No blemishes of any kind on mine. The fit and finish is superb. I have the black one and it truly is a thing of beauty. I really don't want to put a case on it but I know I have to in order to keep it looking like this.
New Lightning port - I can see why they needed to change it looking at the 4S and comparing it to the 5. They definitely needed to reduce the space it took up. I know its a pain particularly if you have a lot of accessories but I think it was a necessary change. New port works well and the fact you can use the cable either way up makes it easier to use.
LTE. Extremely fast. 30Mbps regularly in my area which is a little better than my wifi at home. I know this is not just a feature of the iPhone but nevertheless it is impressive.
Battery, my only gripe. It seems fine so far but I haven't had it long enough to really judge. I just can't help thinking they could have kept it a little thicker and had 20% more battery capacity. As long as it still makes it through the day I will be happy with it.
Camera, seems as good as the 4S with a lot of added speed to boot.
Geekbench. Results are impressive and currently beat anything else on the market including the S3 in stock form. I know it is just an indicator of performance but it shows that this is some serious hardware and is a major improvement over the 4S which was never laggy or glitchy in my experience.
Overall, its a beautiful, speedy, sexy, fully functional device. I love the screen so much I took a lot of apps out of their folders so I can see the icons in all their glory. Is it worth the upgrade? Depends what you have today and what is important to you. I had the 4S and still think it was worth it but for most people I would say if you have an iPhone 4 or earlier model it is a no brainer upgrade. If you have a 4S it is a more difficult decision particularly as it will probably cost more and the major differences are screen size, speed of the device itself and LTE speed if available in your area. I was fortunate and got the upgrade pricing as I have more than one line. My 4S is going to be my girlfriends phone and I will have the new one.
If you are currently using Android and are considering the switch, this is the best time to do it. The iphone 5 is currently one of the best, if not the best, smartphone on the market today and I am no Android hater. I actually have an S2 as my home phone as I am required to use Android and iOS as part of my job responsibilities so I am familiar with both OS's and like both.