Would I replace an 840 EVO with it? Probably. The 840 EVO line was an imperfect drive.
Would I replace an 840 Pro or anything newer than the 850 series? Probably not. Not for performance at least - the additional speed brought by NVME just isn't that noticeable in normal consumer workloads.
If I needed extra SSD capacity and I had a choice between a NVME drive and a SATA drive and they were the same price? I'd get the NVME. Unless I already had one and only have one slot for it, in which case I'd get SATA.
From what I have been reading, the difference from the 970 EVO plus to the 970 EVO is minimal at best. I am in the same position and I am leaning on saving the $40 or so and getting the EVO over the EVO Plus.