Pros: Retina Display, Better camera, LED Flash, Front-Camera, Spectacular yet sophisticated design, build quality way better than previous generation, very snappy, 3G speeds are really fast and iOS balances everything, and almost unscratchable
Cons: Cell reception is really spotty, Facetime (for now) works only through WiFi and somewhat a gimmmick, antenna issues do exist though not a big impact it will affects 3G speeds, and batterry still ends up dying by the end of the day through normal use.
Summary: Well I got my iPhone 4 (the black one and the 32GB version) like a week ago. Let me just say that overall, it's spectacular, amazing, and a stellar device. You cannot go wrong with purchasing the device.
First off, the biggest difference of all is really the retina display, some might say the new redesign is but after looking at the display it's really the biggest difference. If the iPhone 4 used the same display as its previous generation (the iPhone 3GS) the only real difference would be design and overall that's not something I would upgrade for. It really makes your photos burst to life, text are really sharp, and it can playback 720p videos it looks amazing. This is the main feature that makes your old iPhone look obsolete, there is no other feature that does that than this.
Anyways, going on with the other parts of the phone, the home button feels a lot more clickier (to me at least), and the rear camera is great, if not amazing. It's one of the best cameras (on a phone) I've used to date, and the interface is nice, simple, and clean. It lets you choose where to auto-focus on and pictures are taken super quickly (in like half a second it's taken). It also has LED Flash which is nice, it helps sometimes but sometimes it might be too bright and there's no options to adjust how bright the flash is but it's alright. The front camera is great, for it's class (a 1.3 Megapixel camera) and best for taking portrait pictures of your self or for FaceTime calls.
The speakers are well mediocore but amazing for only one speaker. Plays a lot louder than my iPhone 3GS and iPhone 3G. Still I recommend using headphones or a speaker. It also comes with two microphones, one on top near the headphone jack and another on the bottom left next to the dock (it's been switched from right to left from the iPhone 3GS). The two microphones have the noise-cancelling feature to give clearer calls and it does work as it should. Amazing but AT&T's service is the only problem sometimes I would get dropped calls (I've had one drop call so far), I transferred from T-Mobile and was surprised how spotty their coverage was (especially in Orlando, FL).
The second best thing about the device is it's new design. It's really beautiful and sophisticated. The build quality is excellent, it's really hard to think about dropping the device while you're holding it. It feels like a luxury device (which it is but just saying you'll have that "I can't believe I'm holding it!!!" feeling). The antenna is used as the foundation of both glass panels which happens to be the silver wraparound on the phone (which explains the three lines). Another thing is the material they use to build this is almost unscratchable, I have yet to scratch the device after a week's use with no case which I could never do with my iPhone 3GS. The circular volume buttons also add a luxurious feeling to the device and in my opinion better to use than the volume rocker on the previous generations. The ringer switch looks a lot nicer too.
Other features like the Apple A4 processor are nice, makes the phone super-snappy, never slowsdown but it's not much faster than my iPhone 3GS especially since they're based on the same processor (both CPU and GPU). The only difference is the A4 has better clock cycles and power management plus the 4 sports 512MB of RAM which is a nice boost especially for graphics-intensive apps though somewhat overkill.
Like I said, AT&T has really spotty coverage on almost every state, you can't go wrong with buying the iPhone 4 but you can with getting AT&T service. It will depend on your location but for me (in Orlando, FL) I use to be on T-Mobile with my iPhone 3GS (Unlocked and only works through EDGE), it worked pretty well especially considering it was on EDGE. With T-Mobile I had 4-5 bars everywhere, never had any dead spots except for like 1 little spot I encountered throughout the whole year I was with them. I transferred from them to AT&T to try out the new iPhone 4 and at first AT&T coverage was great, but later on going throughout all of Orlando (Kisimmee, Downtown, Avalon Park, Winter Park, Altamonte Springs, etc) I always had a variety of 0-1 bars to 3-4 bars but I could never keep a complete 5 bar status. Still despite the bars being low, I could make calls they were very clear, the 3G speeds were extremely fast ranging from 600Kbps to 2Mbps and it was a big difference from EDGE (200Kbps average).
Another thing is FaceTime only works through WiFi making it more of a gimmick than a real feature. I tried it once and it was nice, works great but like I said I might never use it again.
iOS 4 is great, multi-tasking is done pretty well, not true multi-tasking but otherwise it's just what I need. Things work really fast and snappy, but folders are kinda weird (only 12 apps???).
Overall the phone is amazing, get a case to get better reception (and either way you should to protect your investment), and iOS is just fluid, fast, and great.