So I tried the free windows 10 upgrade a few days ago, the installation took a little longer than it should be, but it eventually got through.
The problem started when it booted to the desktop, the screen was at the wrong resolution and there was no other selectable option in the setting to correct it, then the system would freeze at several minutes intervals, also application icons would just disappear from the taskbar and lastly, the start menu itself would not work, it wouldn't even come up(it did when windows just booted)
Basic system info; Lenovo V570, 64bit, Intel core i5 @2.40GHz with 6GB RAM.