Windows 10 has been out for some time for those willing to give it a try as the Technical Previews have been released on a pretty regular basis.
What you should expect: If you are a Windows 8 user then the user interface you will be familiar with and should have no troubles at all adapting (and adjusting) things to your liking.
For those running Windows 7, you will of course have to get used to the new "start menu" which is really not a big deal, especially since it now does not open in full screen mode as it did with Windows 8.
The best news is that it is completely FREE (for the first year) for any computer that has Windows 7 or 8 and it will be a simple update through the Windows update site, which has already started to plant the seeds within the update site under the optional items. You can learn more about this through MYCE from this post.
Expect the full program to be ready to roll this fall, and possibly earlier--as early as August!
Need more information or have questions whether or not you should be upgrading? Just ask!
