[Windows Phone 8 Q&A, Help & Troubleshooting] GoodDayToDie:
User experience, assuming you're asking about the phone in particular and not the OS in general, is good. The combination of good CPU and expandable storage isn't available in any other Windows Phone model right now. The 720p screen is slightly narrower in aspect ratio than the 5:3 ratio that WP7 used, so apps may be stretched a little unevenly, but even if you look for it it's usually not visible. The screen is excellent, with good visibility in a range of light conditions and great color, plus a sufficiently high DPI that even tiny text is clearly legible. I've had no trouble with the pentile layout; the pixels are just too small to see (and I have pretty good eyesight). Battery life is fantastic, although (like most Samsung devices) it prefers to charge off more power than a typical USB port will provide. Still, if I stream music on it all day long, plus email, IM, web browsing, ebooks, short stretches of GPS navigation, and a bit of gaming, it will still have power left when I get home at night and it will charge up overnight off my Surface RT tablet (a more powerful USB port will let it charge much faster). The loudspeaker, headphone audio, and cameras are all respectable but nothing to write home about, although the double-action shutter button (standard on all Windows phones) works very well. The presence of a physical (not-capacitive) Start button means the phone can be woken from sleep by pressing Start, which is easier for me than hitting the actual power button; for some reason, it's on the side of the phone. The chassis feels durable and is comfortable in the hand, yet it's easy to remove the back plate. It is a bit more slippery than my old HD7, but I haven't dropped it yet. The huge screen size makes typing a breeze and also makes it great for reading text or showing pictures / videos to people.

I've had some troubles with WiFi; be aware that if you need absolutely reliable connects-to-anything WiFi, this might not be the phone for you (I have unlimited cellular data, so if a WiFi network doesn't connect properly I just don't bother with it). Cellular data works well and quickly, and has pretty good reception, but I haven't been able to get the WiFi hotspot feature to work yet (wired tethering works but is a bit of a hack). I have once had to reboot the phone because volume controls weren't working correctly; that was annoying. I haven't tried out NFC yet, but it's present.

Beyond that, there isn't much about the phone in particular that isn't also common with other Windows Phone 8 handsets. You may want to go to a store and play with a HTC 8X or a Lumia 920 or similar; that will give you a feel for the OS. Samsung does supply one software feature not found on all Windows phones: an app to block calls automatically. Their Diagnosis app (used for, among other things, wired tethering) is also Samsung-specific.

Please note that my experiences are based on a SIM-unlocked SGH-T899M handset that I bought from Canada. There's at least one other model of Ativ S that will have slightly different specs, and may work better (or worse) on some other networks.

As for unlocking, that depends on what you mean. SIM-unlocking depends on who it's locked to; I just bought an unlocked phone. The official developer unlock (for installing homebrew apps) is easy enough, but requires Windows 8 and a Marketplace Developer registration (the latter typically costs money but can be had for free by students). It also only provides a limited number of apps. A simpler or more complete sideloading unlock has not been discovered yet, never mind the possibilities of either fully unlocking the device to run arbitrary programs and access everything, or installing custom ROMs. While I'd hope that such things will some, they take time and require a dedicated effort to find vulnerabilities.