the 660p is an interesting one..

it's QLC flash, but uses bits of it's flash as SLC to cache on.

Real world it doesn't seem to affect things too badly, but synthetics or hitting the drive hard isn't great. 

https://www.anandtech.com/show/13078/the-intel-ssd-660p-ssd-review-qlc-nand-arrives/9 

How's the reliability of MLC? I have a couple of SLC memory cards, but they're quite a bit more expensive than MLC, which is more expensive than TLC  and soon QLC. I'd rather stick with fewer bits per cell personally, and pay a bit more.

Linus explains it fairly well:

Apparently, you have to be transferring a lot of data at one time to actually fill the SLC cache and normal usage shouldn't get close to causing an issue.

I am actually in the process of getting a build put together with one of these puppies as the startup drive: https://www.guru3d.com/articles-pages/crucial-p1-m-2-1000gb-ssd-review,1.html

From the review I have linked, it looks like things slowed down once you hit about 100GB in a single hit and I have no plans to hit that limit.

I will let you know if I have any issues with it :)

did you get the newegg special?

Just purchased it from PB along with most of the rest of the parts.

I've used a bunch of these in HP laptops, they seemed faster than normal low end SATA ssd's.

Your one isn't full? That slows them down.