I have not seen any improvements with frame rates or with visual quality, nor have I seen any worsening of these effects.
I installed Windows 10 over Windows 7 and now have the option of booting into either and rolling back to Windows 7 if required.
Cpu usage, RAM usage and VRAM usage seem to be the same as in Windows 8.1 but better than Windows7.
Baseline CPU usage is 1 - 3% in Win 10 about the same in Win 7/8, however RAM usage in Windows 7 is about 20 - 25% in Windows 10 around 14 - 18%, - significant but I did not see any performance gain!!

All my drivers, printers, video cards, network, etc have worked OK without issue.
Overall the desktop has been tidied up, and if you know ins and outs of Windows 8, then you will find Win 10 easy, much more like Win 7.
STEAM has not been affected in any way and I have made purchases of new DLC without any problems.
The OS is stable, no crashes, BSOD, etc - it looks like (when released finally) it will be another OS improvement over previous versions.
Just a few notes in case you were thinking about trying Windows 10 - its free and painless.
Regards
pH