Share your experience!
In Sygic and Wisepilot, when i turn the phone horizontaly, like real GPS navigator, that damn phone turns the cardinals. When im turned to north, it shows im turned to east. When it is in verticaly position it is ok, but... Im pretty mad! Nothing works in this damn phone correctly. The camera, GPS, Music, Yellow spots... I just want my money back and never see SONY again! Really, that Xperia S is just a big problem 
Sorry to hear that you're not satisfied.
Did you try to run the compass calibration?
Go to the service menu.
Enter *#*#7378423#*#* in your dialer > Service tests > Compass.
If that does not help pehaps you could try running a software repair via PC Companion.
Delacroix,
What version is you firmware. I had the same problem while on .62. After updating .73, the problem is solved.
I have .73. And i dont know if it is normal, but when i turn the phone horizontaly on the main screen with widgets or in the list with all apps, the screen does not turn, it is still set as verticaly. All apps are turning but the main screen not.
Does the software repair delete game save data? Or maps in navigations? Or all applications itself?
if ur'e saying your phone doesn't switch bet ween potrait and landscape, go to Settings > Display Settings. Check if "Auto-rotate screen" is enabled. If it is, then i suggest you do factory reset, or run the software repair like what Johan suggested. It's been years since i used that so I'm not sure if your data will be over-written or not.
If doing this still doesn't fix your problem, then it could be a hardware issue. U should return your phone if that's the case.