Samsung make absolutely great drives but when they f*** up, they do it massively. The 840 Evos had the degradation problem indeed, they did get a firmware update that temporarily fixed them, but they'd just go back to the same issue over time. Afaik there was never an actual fix.
The 850 Pros once got a firmware update that bricked the entire drive, the update was removed within a day but it was still a little awkward for Samsung.
Tbh, if I were to buy an SSD right now I'd probably just get another MX100. All SSDs seem to realistically max out Sata III speeds by now anyway, so one really isn't going to be much faster than the other, it's more of a case of reliability (but don't get me wrong, overall Samsung are great for that).