This seems to be a pretty good, common sense article on whether you should get the win 10 freebie.
I have an XP "Era" computer that I put win 7 Pro 32 bit on but it just doesn't make sense to bother with upgrading it. My other computers on the other hand would handle it but I need to decide if it is worth it. I actually bought a computer to get the upgrade on to see if it's worth putting on a couple of others in the house.

"Windows 10 is coming on July 29, and it's going to be free for most home users of Windows 7 and 8.1 until July 2016.
But while a legitimately free copy of Windows is pretty enticing, upgrading isn't the best move for everyone.
You may recall that Windows Vista had some compatibility woes, largely due to how other software detected the version of the operating system.
Depending on the vintage of your PC and how you use it, upgrading your OS may not be worth the potential trouble, even if the OS is free.
Let's look at a couple scenarios where you might want (or not want) to take the plunge into Windows 10."