Enough is enough. I got sufficiently annoyed by Samsung Android to install an unofficial port of CyanogenMod (no better alternative) on my phone. What are the first impressions?
The system installed without problems and has been running reasonably well. The user restrictions are gone: I got rid of many useless proprietary applications wasting the very limited space on the internal storage, some things got customizable and root access is available when needed. There is improved functionality: I especially like profiles and swype on the Google keyboard works much better than on the Samsung one. Software freedom was improved by removing some pieces of unwanted proprietary software and replacing some components of unknown origin and license. While I miss a few things from the original Samsung system, I absolutely don't regret abandoning it and have no intention to return to it unless I experience some serious problem.
As for stability, neither of the systems is perfect. Samsung system suffered from random reboots and other random stability problems. CyanogenMod has problems to start on my phone, suffering from boot loops, but once it's completely up and running it seems to be stable (so far). Time will tell but it seems the Samsung official preinstalled system isn't more stable than an experimental unofficial port of an alternative ROM.
Samsung produces user friendly hardware: replaceable battery, SD card slot, standard SIM size, a lot of different models for different needs, available bootloader. Too bad they cripple it with their proprietary software. I'd probably recommend my friend buying a Samsung phone, but only one of the models for which one's favorite alternative system (e.g. Replicant, OmniROM or CyanogenMod) is officially available.