Today I had some time with Windows 10, and I was amazed how well it handled the migration, everything worked on the first go.
On my work PC I ran into the problem of enabling Cortana.
My option in Cortana menu was greyed out. So I started to look into the issue, and it turns out Microsoft has released Cortana only for certain countries like US, England, Australia, Canada (English only), India (also English) and Japan.
Since my country was not in the list, I had to follow these steps:
TIME&LANGUAGE -> Region&Language
Now this is very important you need to Select BOTH Country and Language (set to main windows display language) from the same Area.
Like here both UK.
Now you are able to Turn On Cortana, just be aware that you need also a Microsoft Account for her to work accordingly.