I've had a week (give or take) with the first two Samsung Galaxy S phones for US carriers, the AT&T Captivate and T-Mobile Vibrant. What do I think so far?

• The two phones are very similar in terms of functionality, features, and software. The main differences are the carriers and the design/form factor. T-Mo's Vibrant sticks close to the original Galaxy S' look and feel, changing only the front panel buttons, while Captivate sports a relatively radical redesign.
• Personally I like Vibrant's look better, but AT&T gets an A for Effort in at least trying to give their Galaxy S its own personality. 
• Vibrant is either fantastically thin and lightweight or horribly cheap and plasticky-feeling, depending on your point of view. Its rounded-corners rectangular body with chrome trim is pretty generic looking but feels nice in the hand. Vibrant is on the left in the below photo.

• Captivate sports squared-off edges and corners, a trick battery cover mechanism, and a textured back panel that feels like metal but apparently is made from a composite material of some sort or another. The overall effect is a phone with more heft than the original S, once that feels more substantial in hand without being at all heavy. But I don't much dig the sharp corners nor the style of the three-tones-of-grey back of the phone.
• The displays on both phones are fantastic. Super AMOLED really pops, and while it's hard to say if these are "better" than iPhone 4's screen, colors definitely leap off the screen in good lighting. T-Mobile included a full-length copy of Avatar with the Vibrant, and the little bit that I watched looked fantastic.

• Performance has generally been very good on Vibrant. I got a buggy Captivate, and Samsung has sent out a replacement for me to test - apparently a small number of Captivates are plagued with some gremlins in the front-panel touch controls that wreak havoc on the system. My device kept auto-triggering the Back and Home buttons to the point that the phone became entirely unusable. Thanks to all of the readers who commented that they had similar experiences but also had no problems swapping their devices out for new, bug-free units at the point of purchase.
• Swiping, pinching, zooming, tapping, and otherwise navigating around Android 2.1/TouchWiz 3 has been smooth and almost entirely snappy on Vibrant (and Captivate, when it was working). While I've encountered a tiny bit of lag while launching apps, it's been the exception and not the rule. Web page rendering and zooming has been on par with best-of-breed smartphones, and typing has been quite nice thanks to the spacious 4" display. Both phones come with Android, Samsung, and Swype soft keyboards pre-loaded.

• The accelerometers on the phones seem a wee bit over-eager, or almost twitchy. Take that one with a grain of salt until I get a replacement Captivate and/or spend more time with Vibrant. I carried the Captivate with me all day Friday and Saturday before the button bugs really took the system over, so my experience with the accelerometers may have been tainted.
• Both devices "feature" a small number of custom carrier apps that could be considered bloatware. T-Mo's software will likely be received a bit more warmly, as it includes Sims 3 and Kindle in addition to the aforementioned Avatar movie file.

• Both phones are thin, powerful, and seem to have decent battery life as compared to the other "superphones" on the market. My first take is that the Galaxy S line acheives better battery life than Snapdragon-powered rivals from HTC. We'll see if that holds true over time.
• I don't find TouchWiz 3.0 offensive, but I know some Android fans who do. The skin can't be turned off, but the individual widgets can. That said, I like Buddies Now and Daily Briefing. The rest I can do without.
• I just shot some HD video and am uploading the raw file to YouTube. Stay tuned!