AVOID. I'm serious.
This is why.
I've installed 10.0.12534.52 just yesterday. It was the first time Windows managed to install Windows 10 build over my Windows 8.1 GDR 3 DP (Developer Preview). All previous updates were failing to install (I believe due to insufficiently large EFI partition which is by default 100 Mbytes on Samsung ATIV S instead of required minimum of 128 Mbytes).
Everything (except for 80% of apps) worked fine and I could call on somebody and receive calls up until I decided to change my region, an operation that requires a restart. Since then the phone lost my SIM card and I can't get it to rescan the module to detect the SIM.
/* Rumors say, if your SIM is enabled with PIN, you could cancel entering PIN and change it in Settings|Phone, but my SIM doesn't have a PIN set on it and I don't have the factory default PIN/PUK with me. */
In one day the phone filled my ?SD card with unknown stuff so that there was no space left on it to the end of the day. Out of 3 Gbytes of free space I've had when I installed the build.
Hoping for better yet, I installed the 10.0.12534.59 today and... the first issue I encountered was the cyclic restart of the phone appeared when updated completed to install. Solution for me was to pull out ?SD card.
Summary:
1. You cannot make calls: the phone does not see the SIM after you restart OS. That's a burden.
2. None of navigation apps work except for Yandex.
3. None of the dictionary apps work. Totally borked. They keep shown in the task view but when the focus is returned to the app, Windows hides the app and shows the Start screen.
You'd better off installing Windows 10 on ATIV S for now. Possibly some of the issues are due to obsolete firmware that I have on my ATIV S, but I didn't manage to obtain the official update from Samsung that was issued in October 2014. Yes, I have ATO (Austria) region set in my registry, yet that didn't help.
What firmware do you have, BTW?