Any point in waiting for Samsung 850 EVO?
Ask yourself this, do you need a performance boost now or can you wait an unknown amount of time for a possibly unnoticeable difference between the 840 and 850 EVO?

I personally have an 840 EVO, install them all day long and before my EVO I had an Intel SSD. EVOs are great but the decision should be as simple as do you want it now or do it you want it later. No matter what you won't regret your decision.