I just thought passing by and leaving a message over here, as I'm a 840 EVO 250GB owner for quite some time now - August 2014 and I've been through the process and frustration going on with this drive. Fortunately, I was patient, and judging by some of the opinions and facts presented here also a bit lucky to now have a full-speed working drive, which finally became so after the latest (D) firmware - somewhere in April 2015.
I also followed this thread for a lot of time - here is where I discovered the bad situation with Samsung EVO 840, so I want to thank all those who contributed with meaningful information and analysis!
All in all, I wasn't so mad at Samsung for the situation, so that this year I bought a new 850 EVO 250GB for a laptop PC! After aprox one year of existence on their side and no peculiar behavior like with the EVO 840 I felt confident enough to finally give it a shot - and at the limited market offer I have in Romania, the best price-quality products seemed to be Samsung's.
But to expand a bit on the 840 EVO "saga" as it was experienced by me.
After initial purchase at the end of August 2014, it all went well for a while, but this is a HDTune pass from December 2014, with no intervention what so ever on the drive (refreshing of written data etc.):
By January 2015 I started doing the "restoration" offered through Samsung Magician, which basically refreshed written data, so that it would even up performance:
This didn't last long - as after some months - the pefromance degraded, as reported by others as well - so that here is how it looked for me on the 11th of April 2015.
Shortly after that, in April, the D firmware finally came in - which fixed the performance problems on reading data.
It really worked for me, and after it, I only got small spikes in read performance, like you can see in the image from 26th of November 2015:
Nothing worse than that - but this is a consistent performance, so since the latest firmware upgrade I can consider myself content of the results and handling of Samsung - although it took a lot to address the situation and who knows with what hidden/underneath cost for the SSDs. Fortunately I still have a one figure write TB value after an year and 5 months - so whatever extra writes may be done in a hidden way, I don't think it would compromise the drive too soon.