If you like black levels, a snappy response, great call quality, and ease of use, this phone is your best bet. It is also very light weight, has great battery life, and a better than average screen size.
I became a fan of Windows Phones over time. I have given myself a year to try all OS's and different manufacturers. I like several OS's, but I just love the live tiles and Metro UI of WP. The Samsung Focus S is my favorite device going right now. This operation is seamless. From scrolling, to loading up of web pages, to launching of games and media, the "S" just performs. That AMOLED Plus screen is just alive and vivid, as you would expect a Samsung screen to be. The 4.3" screen just bursts with color, and I love the way the live tiles just explode off the screen. I am not a big fan of the auto adjust on the screen brightness, though. Often it over adjusts. I just set it to medium. There have been complaints of a "blue-ish tint" to Samsung screens. I have seen none of that, making me believe it may happen under bright settings. There is very little delay between switching between portrait and landscape, The onscreen keyboards are okay. I find myself using the portrait layout much more often due to the WP still unable to figure out how to get several screens to work in landscape mode.
The call quality is very good, although I am not a huge fan of the speaker phone. I have found it to be a little distorted even at lower volume. No such problem with it's normal speaker. It sounds very strong and clear. Reception is great, and holds the "fake 4G" AT&T network just fine.
Build quality is fine. Unlike some reviewers, I find the textured back great for grip. I love the button layout. I am a little tired of the pictures of my floors and inside of my pockets that happen from accidental pushes of the camera button. But it is a nice option, and I would miss it now that I am used to it.
The camera itself is fine for casual pictures. But it is much stronger outdoors than indoors. It's edit options are just a basic set of functions. It's macro (close in) details are fair. The single flash is fine, but seems to wash out details in really dark settings. Shutter speed is not great. Overall, I would say if photos are a kill point for you with a device, you may want to consider the HTC Titan which has a much better camera.
Multimedia plays fine. Audio is okay for music. The Zune player is outstanding. But keep in mind two major drawbacks: The first is Microsoft's completely baffling decisions to not allow expandable storage or not to encourage manufacturers to build devices with 32 GBs of storage. With everything loaded on the device, you have about 13 GBs of usable memory. That is not horrid, but it won't allow for all the goodies a lot of users want. The second is that if you frequent a lot of Flash driven web sites for video, WP is not equipped for that. While the industry is moving away from Flash, only Android supports flash players. The GS2 is a wonderful option if you need flash and is a great device.
Battery life is very, very good. You can get through a good 8 - 10 hours with heavy usage.
I've just upgraded to the Focus S and I'm sold. I was going to test it out for a week until the Titan comes out, but now I just may keep it.
Unless the Titan blows me away, the Focus S is definitely the next step from the Focus. I'm glad Samsung is staying around manufacturing WP's.
Warning: Phone is very thin and slick, can slip off hands. Takes a bit of getting use to. =)