Been using High Sierra since the first DP beta, and it's been pretty good so far. The early builds were definitely rough around the edges (to be expected), but the past few builds have been relatively stable. I've noticed a few issues, mainly with iTunes and Photos. When I have iTunes open the computer can slow to a crawl - but not always. I haven't tested it since the 12.7 release of iTunes, so that might help. Photos is just buggy as hell. I have a relatively large photos library (167.7 GB, 18000+ photos and videos, mostly photos), but had few issues under Sierra. There were periods when it would slow down some, but usually when I was importing a bunch of photos while editing existing photos and what not. In High Sierra it can run really slow all the time, in full screen mode I get weird screen artifacts (like a white bar on top, where the toolbar should be - until I mouse over it and the toolbar appears), can crash a lot - typically I get the spinning beach ball of death and have to force quit. I've reported the various issues to Apple, and while Photos has definitely become more stable since DP1, it's still buggy. I'm actually really excited about some of the additional editing features in Photos - will require less use of 3rd party extensions - but at the moment it is rather annoying to use. Hopefully, though, these issues can get ironed out before public release (haven't had a chance to install this GM candidate to test it out).
My biggest cause for concern, however, is APFS. I'm currently running APFS on both my MacBook Pro with dedicated flash storage, and my iMac with a fusion drive. High Sierra automatically updated my MBP to APFS, but did not updated the fusion drive. In fact, I could even manually convert the fusion drive to APFS - I actually wiped my machine completely, recreated the fusion drive and formatted it as APFS (so obviously APFS works with a fusion drive). So far it seems to be running just fine - and there are some obvious benefits to it already. I had a major issue in a recent build that prevented my iMac from booting, I figured I'd have to just restore from a Time Machine backup. Fortunately, because of APFS, Time Machine creates local snapshots on the internal hard drive, and I was able to restore to one of those snapshots prior to the issue I had. It was easy, fast, and actually worked (unlike a similar feature in Windows allowing you to restore the system to a previous point if you're having issues, which rarely works to resolve the issue). With that being said, and speaking of Time Machine backups, external Time Machine backups still need to be formatted as HFS+. It thus appears that at the moment, external Time Machine backups gain no benefit from the new features in APFS (specifically the aforementioned snapshots). I'm sure this will change at some point down the road...but who knows when. Which brings me to my biggest concern about APFS, there's just not a lot out there about it. Apple has published a number of different documents delving into some of the features and specifications about APFS, but there are still a lot of unknowns. The fact that there is no mention of external Time Machine backups in the documents (only local backups are mentioned) and that a lot of API's haven't been published (snapshots, clones, etc) just has me leery, it feels like APFS is still in beta stages. This wouldn't necessarily be an issue, but any machine running just flash (no fusion drive) will be automatically upgraded to APFS, with no option to NOT be upgraded. Hopefully when the final release is available Apple will publish more info on APFS.