It kept everything of mine. I did have some issues with a few different videos weren't able to be played because it was saying it was compressed in a way that they didn't recognize. I updated my VLC player and everything seems to work (even though I've been watching with windows media player)... I also had to go and change all the default "open ____ file with ____" but it was actually really easy, under settings and took about 5 seconds to change everything.
I also got minimized the search thing so it doesn't take up half the task bar (I may dump it all together, not sure yet), and got rid of the task view button, I'll never use that. Organized my start menu to my liking (I really like how they have that set up, one of the better things about it) as well.
For some reason my antiviris had some weird issues after updating on both computers. I use bitdefender, which is compatible with Windows 10, but both times when I upgraded it got lost or something. I couldn't find it, but when it was still on the computer. I ran the installer again and just hit repair, and in about 2 minutes it was good. Spent about 30 minutes running an update and everything is well, so it's running smoothly. I usually don't geek out over OS's, but so far I feel they've knocked this one out of the park. Maybe it's just because 8 was such a pile of crap it makes this look that much better, but I really haven't found anything I dislike about it thus far.
