I've had my iPhone v4 for a week now, so I thought I'd share some brief thoughts here.
Reception
I've had no real issues with reception, in spite of all the news stories on this topic. I think a lot of my good experience is related to my paranoia - I ordered a case for my iPhone before I even received it, because I was worried about it being too slippery, and wanted some protection from scratches & drops. This meant that I avoided the reception drops reported by others, since my fingers don't touch the metal rim directly. 
The case I chose is Boxwave's Cyclone Crystal Slip (in Azure Blue, pictured at right), and it covers the metal band around the iPhone, as well as the back. I test dropped my phone with this case, and it does an awesome job of protecting the phone.
Form factor
I upgraded from an iPhone 3Gs, and the iPhone 4 is very similar in dimensions, but slightly thinner. This makes it more pocket-friendly, and it feels like a very sturdy little gadget. The glass back makes it very slippery so I definitely recommend some kind of case to help you grip it better (again, the Cyclone case comes to the rescue).
Battery life
As I've posted here in the past, my biggest gripe about the iPhone has been battery life - with the iPhone 3Gs, I had to surround myself with chargers and plan recharge time into my daily routine. With the iPhone 4, the battery life still isn't breathtaking, but it's notably better. With normal use and identical settings, I can get through a whole day without recharging. Might not sound like much (especially compared to my old Blackberry which could go for a couple of days under heavy use), but it's "better enough" for me.
Screen
The screen is awesome. At first I thought "yeah, whatever" but I am really noticing how much better the higher res screen is, even in non-graphically intensive apps.
Camera
The camera is fantastic. The resolution, of course, is about 5x better and the picture quality is great even in low light. In really low light, the flash kicks in and does a nice job of illuminating your subject. There is also a digital zoom, which can occasionally be helpful (but digital zooms sacrifice picture quality).
Facetime
One big change is that Apple's added a front-facing camera (1.3 megapixel) that allows you to take pictures of yourself (whee...) or have a 2-way video chat with another iPhone 4 user. It works pretty well, but the iPhone 4 requirement limits how much you can use it -- and the fact that it currently works only when you're using Wi-Fi limits it even further. I'm looking forward to when they allow you to make these calls over the 3G network, and when they enable Skype or similar video communications apps to use the iPhone 4's camera.
iOS v4 improvements
The new version of the iOS (previously known as iPhone OS) is pretty cool. My favorite improvements:
- You can now create folders of apps to help you organize your apps. It's about time!
- Email is much-improved, allowing you to show all your various email accounts in a single inbox view.
-
Bonus: Mail also has a "conversation" view which collapses all the messages into a single entry in the inbox. When you click the message, you "drill down" into a view that shows only the messages relating to that specific subject.
The negatives
All in all, I really like the iPhone 4 and am glad I upgraded. But it isn't perfect. Here are my gripes:
- Still no support for Flash content. The religious war between Steve Jobs and Adobe make this unlikely to change.
- The A2DP Bluetooth still doesn't support the [Previous Track]/[Next Track] functions, caller name announce, or really anything beyond [Play]/[Pause].
- I still wish the battery life was better. If the thickness of the iPhone 4 was the same as the 3Gs, but I got longer battery life, that would be no problem at all for me.
- Multitasking is not very impressive. Maybe I just need to be patient for more multi-tasking ready apps, but it's not all I'd hoped. Its really more of a "multi-pause" capability at best - aside from a couple of apps (Pandora, for example) it just pauses your apps in the background so you can restart them faster.
You may notice I didn't mention AT&T. That's because I don't have an issue with AT&T - their coverage in the Portland, Oregon area has been solid for me, I travel internationally where their roaming works fine (though Int'l roaming can be ridonculously expensive), they let me keep my unlimited data plan. Plus, I like having an iPhone.