I've been playing with my iPhone for a little bit, here are my quick, unorganized thoughts.

• I bought a bumper, and before I put it on, it felt like the phone was incredibly droppable. The tapered edges feel like they want to slip through your fingers. I highly recommend the bumper.
• The plastic back on the 3GS has a slightly higher coefficient of friction (less slippery) than the new iPhone 4 back which I assumed before I got the phone. Not slippery, but definitely slipperier than my previous iPhone.
• The camera is astounding. I took a quick picture of a flowering tree outside the office and it was incredibly sharp and beautiful. Probably the nicest picture I've taken in years.
• It's thinner, about the same size as just the plastic case on the 3GS. Slightly thicker than the iPod touch which is like a wafer.
• The screen is making my eyes buggy because it's so sharp. I've never seen a screen with this type of pixel density and it's so sharp it's almost jarring, like my eyes aren't used to it yet.
• The screen is brighter and has a different color profile than the 3GS. It's a lot more contrast-y, blacks are blacker, whites are whiter. The angle at which you can view the screen is incredible due to the new IPS display.
• Text rendered on the new screen looks foreign it's so sharp. I've never seen Helvetica like this. Viewing text on the display is now my favorite thing to look at.
• Apps that haven't been updated with 2x-resolution raster images look like shit; pixelated and blocky and clunky. I didn't think I'd notice but I definitely do, it's unmistakeable which ones aren't optimized for the new screen.
• The fast app switching feature in iOS 4.0 makes the phone feel way faster than the 3GS. Obviously if you load iOS 4.0 on your 3GS it'll seem faster too. I think this feature is biasing my overall feeling of quickness on the iPhone 4, but everything seems much faster. Apps feel like they want to start displaying their interface before their opening image is done zooming out. Reminds me of when apps open really fast on a Mac, before their first bounce is done.

So far, it's brilliant.