Some commentators [1] on other sites are saying that the new Filesystem can't cope with BitRot which happens with Solid State devices. It has been many decades since I was involved with the innards of filesystems so it would be nice to know if this is a real issue or not. Has it gone away with modern hardware?
[1] ZFS diehards mainly.
So-called bit-rot is no more common on SSDs than it is on spinning platters. Corruption happens, sure, but nowhere near as often as the ZFS diehards think it does.
It's more common on RAM. Reboot, folks!
One additional thing for rabid "bit-rottweilers" to consider: If bit-rot was such a perversive issue with modern SSDs, and Apple is the largest consumer of NAND, which they use in every device with an OS X variant, then why did they not consider addressing this widespread issue in their new file system?
My guess? The same reason the original Macintosh File System in 1984 wasn't hierarchical; there wasn't time. Apple has a "done is better than perfect" approach, and it's not uncommon for version 1.0 of something not to have all the features you want. I'm still holding out hope that APFS 2.0 will be checksummed.
Regardless of all that, this is still a huge, huge step forward, with or without checksums.
I don't know enough about filesystems to know whether that's a requirements, but I have read that they started working on APFS two years before they announced it at last year's WWDC, and I know from a long history of studying computer networking that many features in the lower levels of the networking stack are deprecated because it's more efficient or redundant when done in the higher levels. Of course, the opposite can be true in networking and lower level checks can be less resource heavy, so take my comment as its intended, and not as an implication that checksums of file data—not just the metadata—isn't necessary.
The reason checksums are nice is that you can run a scan on a disk, and know with 100% certainty that all the data on it is intact and undamaged. This is a really, really nice thing to know, for instance, on your Time Machine backup drive (and conversely, it's nice to know if the integrity of your TM backup isn't intact before you end up wiping your main drive and restoring from it). Apple's claimed that the ECC on their SSDs is fail-safe enough that bit-rot shouldn't be an issue (http://dtrace.org/blogs/ahl/2016/06/19/apfs-part5/), but Time Machine drives can be on any kind of hard drive at all, and it'd be nice to have this peace of mind. I don't see any reason why it couldn't be added in a later version of APFS, though, as it seems like a pretty additive feature. So for the time being, I'm just delighted to finally see a new file system for macOS.
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