I have Windows 8 and haven't had any issues (other than the normal FSX ones). As for adding textures, I know you can't edit aircraft config files and save them because of UAC, which can't be shut off as easily as with Windows 7. I typically just save the file to my desktop, and then move it to the aircraft folder and click "yes" to replace, that works.
Flight Simulator X Gold Edition (REX Essentials +)
HP 2311xi 1920x1080, Windows 8 64 bit, i5 3450 @3.5 Ghz, GTX 650 Ti 2GB GDDR5, 8GB DDR3-1600