Quite possibly the best and worst thing about the Samsung Focus S is the fact that it is a Windows Phone 7 phone, and running the latest version, too (7.5 Mango). The greatest thing about WP7 is that it really is nothing like the competition (ie: Android and iOS).
Of course, the flip side to that coin is the massive learning curve you'll run into if you're coming from anything that isn't WP7.
But don't let that discourage you. WP7 has a handful of unique things that it does extremely well by integrating just about every feature imaginable. Uploading a photo to Twitter or Facebook? Integrated into the camera. Want to identify a song? Integrated into Bing Search.
The real discouraging thing is everything that you take for granted with your iPhone or Android phone that Microsoft left out, things like a system-wide file manager, or even the ability to upload arbitrary file types.
As long as you can look past that, WP7 still offers a very usable system. The Metro UI, while a little plain, does grow on you quickly. It's all about the "tiles."
Tiles are sort of a combination of icons and widgets. They're fairly small, like icons, but have the capability of serving up dynamic content, like widgets. You can reorganize the tiles however you like, pinning or unpinning with a simple long press.
Customization doesn't go much further than that, though. It's possible to change the background color and primary color of the tiles, but that's pretty much it. It's a step further than you get with iOS, but considerably more limited than Android.
Apps are located to the right of the tiles, listed in a single vertical column. WP7 does feature its own app Marketplace, but again, if you're coming from Android or iOS, you'll be a little disappointed with the selection, at least for the time being.
Microsoft has chosen to integrate the Windows Phone Marketplace with Xbox Live, so there's a good chance that we'll be seeing a competitive gaming market in the not so distant future.
The Focus S also features multitasking, as do all WP7 phones with the Mango update, but it's far from perfect. The system seems a bit too eager to kill background tasks, even if you're not doing anything else.
For most apps, especially games, leaving them in hopes of coming back and picking up where you left off is always a gamble.
Ultimately, if you're a WP7 veteran, you'll have no trouble finding your way around, but if you're coming from anything else, you'll probably find yourself cursing at your phone a bit in the first week or two.