Much better from 3GS but certainly an Apple 'trap'.

iPhone 4 is thinner, shorter and ever so slightly heavier than iPhone 3GS. Gone is the curved plastic back plate, replaced by a flat glass panel and a shiny metal band that wraps around the perimeter of the device. This is the first major redesign since the original iPhone debuted three years ago. The basic design - touchscreen and single front-panel button - is the same as it's always been, but the styling cues, accents, and overall hand feel are markedly changed.Most striking is the screen, which has a new sharpness from its special 3.5-inch Retina display that gives a 960×640 resolution – the highest of any smartphone. The camera, featuring 720p HD video recording, appeared to be much better quality – it has a five-megapixels sensor, compared to three on the 3GS, and features an LED flash that also doubles as a video light when recording.
Running the new 4.0 software and with an A4 Apple processor inside, response times seemed markedly faster than its predecessor.
If you’re a gamer, you will appreciate the addition of a gyroscope to the iPhone’s motion-sensing capabilities, and if you are just a general user, improved battery life that allows up to 10 hours of internet browsing on Wi-Fi means the iPhone will still be lit up and functioning long after the Evo, with its weak battery life, will have shut down.
On the bright side, FaceTime video calls are fantastic, likely because there's no cellular signal involved (you're limited to WiFi calling). Video and audio are great, it's easy to switch between front/rear cameras and landscape/portrait orientation in the middle of a call, and the narcissist in me loves flicking the preview window from corner to corner instead of paying attention to the person I'm FaceTiming with. Apple was incredibly smart to make video calling "just another button" in the standard phone interface, making it much easier to place a video call on an iPhone than a comparable Android device. The limitation with FaceTime, of course, is that for now you can only make video calls between two iPhone 4s. But that's kind of the point, from Apple's perspective.
Usually I wrap up phone reviews by saying - or at least thinking - that a week or two with a new smartphone is hardly enough time to get to know it, let alone be able to tell how it'll hold up over the long haul. In this case I've got the opposite feeling. By now all iPhone owners are familiar enough with the operating system, the iTunes/App Store ecosystem, and the way an iPhone works that a few weeks with iPhone 4 is plenty of time to get to know the thing. The question that remains isn't how well Apple's new phone will hold up over the long haul so much as whether or not Apple will be able to fix the issues that are plaguing the device right out of the gate, or if they'll wind up as part of the cost of being an iPhone user.