I`ve recently "upgraded" to an 500Gb 850 EVO. The 840 EVO has gone into a new laptop, which won't be getting a lot of use, and could be left switched of for weeks, or even months. It'll be interesting to test the 840's performance after a few months without power.
Meanwhile the 850 seems no different in performance to the 840 in my PC. A bit more spare capacity though !
My 840 in my laptop is down to 400mb for most of the test in hdtune now Laptop doesn't get a lot of use.