I made a 2nd APFS partiton (so, shared space with my default High Sierrra partition) om my 2017 iMac / 32 GB RAM / Radeon 580 8 GB / 1 TB SSD, and installed this first beta "clean".
Tried the dark-mode immediately as the setup assistant gave you the choice (see pic)
At first glance I liked dark-mode. Finder, System Prefs, etc. all looks pretty cool IMHO.... until you open Safari and go to macrumors.com (or any other site..) as the web page is full white. The contrast annoys me hugely.
Unless all "white" pages can be turned into dark mode, this dark-mode is just a simple useless "theme" at the edges of the application.
Speed seems the same as 10.13.5 (only tried X-Plane 11 so far) and no immediate issues with stability, but haven't tried it enough for good testing.