I won't be updating to the free version but I cannot say whether or not I will end up using it at some point in the future. My general rule of thumb for computing is that I only upgrade or update based on requirement. Either due to if I need to run a software program or part failure.
The "free" version is advertising based. You will get advertisements in the Live Tiles in the Start Menu and likely will see advertisements in other parts of the OS. Placeholder ad spaces were found early on in the Insider Preview. The warranty or support for the OS is based on a general "lifetime of device" estimate. For a Desktop PC this is 4 years from GA, July 19, 2015. They have no concern whether your PC is one month or 10 years old.
In the OS in general (not just the free upgrade) there are some big time privacy concerns. When you go through initial startup, it asks you about settings, with a "Use Express Settings" button. If you read the text there you can see what it does. If you choose customize settings it breaks it down some more.
Whether you are using a local or Live account, you can almost guarantee that your info is going to be farmed out eventually.
Just about all other concerns I have about the OS is mainly UI related. There is a lot of empty space as the OS is still primarily designed for use on mobile devices. Search has been changed so it will search online as well as local to the system. The search bar takes up too much space and search has been bad since Windows 7 for my needs. Third party tools can fix this.
Another thing is you cannot block updates. This Windows Update will try to install drivers. Anyone who uses their PC for gaming will already know to not ever install the video drivers from Windows Update. This is especially a problem for anyone who needs to run specific driver versions for their software.
There are some things I do like about Windows 10 that I wish were in previous versions. One is the recovery/refresh mechanism no longer requiring a separate recovery image. However there is a concern that this may end up being insufficient to removing an infection, since Windows 10 uses the backup files for refresh. One problem with doing a Refresh is that it will uninstall nearly all of your software. It isn't a repair install like in the XP days.
If I were to use this in the future, I would add it to a domain. And then have WSUS installed on the DC in order to handle how Windows Update works and then I can control what updates get offered to the PC. And of course shut off all those options I posted in the pics in the spoiler. And finally would use a Start Menu replacement such as Tihiy's StartIsBack because the Start Menu in Windows 10 suuuuuuuuucks.
