Been using it since yesterday. Yeah, I live dangerously. So far, mostly a good upgrade. I like the new look, modern with a reasonable level of skeuomorphism so I can still feature out the functions of the icons.
Tested: MBP 2012 (16GB RAM, 512 GB SSD) and iPhone 5s
Some features are just not working for me:
* AirDrop from iPhone to Mac. I can AirDrop from Mac to iPhone, but my iPhone can't see the Mac.
* Placing a cellular call from the Mac, I can't hearing the ringing. But it works just fine otherwise.
* it took iCloud Drive awhile (1+ hour) to show up properly on the Mac
* still doesn't show my network transfer speed. Occasionally, I'm moving 5+ GB files between my computer and NAS, I would like to know if there's a bottleneck somewhere.
Too smart for its own good:
* short-hand/text expansion you set up on your iPhone will automatically transfer over to Mac OS X