Its kind of free but kind of not free as far as I can tell.
Right now I have 2 laptops and 1 desktop running Win 8.1. They are all eligible for upgrade to Win 10 free on July 29th. My understanding is that the Win 10 upgrade will grant you a free copy of the O/S that's good for 'the lifetime of the machine'. So in the case of my laptops that's probably fine as I doubt I would ever be changing the MOBO or other components sufficiently to make them 'a new machine'. So the installed win 10 would likely last beyond the first (free upgrade period). Its alway possible that a laptop component would go 'pop' and a new drive or something would be needed and in that instance I don't know what happens to the Win 10 upgrade.
My major concern is my desktop. I change parts in that fairly regularly. I have 5 HDD and an SSD for a start, I upgrade them quite a lot, I have changed CPU and MOBO in the lifetime of this machine. I am seriously considering a change to Skylake when it arrives so thats potentially a new MOBO, CPU, memory right there.
My question is if I go ahead in July and install Win 10 either as an upgrade or a clean install from media (if thats possible) what happens if I opt to move to Skylake in say November odd? New MOBO, CPU etc so I assume that's the lifetime of this machine over, so does this mean I couldn't even reinstall Win 8.1 then upgrade to Win 10 again because I have already had my free upgrade once? What happens if I wait to change my MOBO until the win 10 'free' period is past? Do I have to buy a new copy?
It seems suspiciously to me that what your actually getting here is a one time upgrade to a free OEM version of Win 10. So my Win 8.1 Pro full retail version gets traded in for an OEM copy, not a proper retail key. Not really too happy about that idea. What I want is the Win 10 equivalent of the O/S I have now i.e. a Pro version that I can clean install from media(USB/DVD) and I can do that as many times as I like regardless of any changes to hardware I might make.
It seems to me the 'free' aspect of all this is minimal if you are a serial upgrader tinkerer with your system. If your joe public who buys a laptop from PC world and can barely change a hot swap drive let alone a MOBO then this seems like a good deal (at least until the hardware failure that leaves you needing to buy a new copy of win 10 on top of the cost of your replacement HDD!)
My own instinct right now is to look and see if a proper full retail version will be available for reduced cost rather than rush headlong into a free scenario thats so full of questions.
By the way this isn't rocket science and Microsoft could and I am sure has already thought of these questions so why don't they answer them straight away when they announce? The MS boards right now are full of the same questions, can you install clean from media?, can you move the win 10 install after a hardware change? 10,000+ people with the same questions but nothing from MS I wonder why so reticent?