I think it is worth it because if yiu buy a few Win 10 KEYS on ebay they will be usable a few years from now unless the seller is dishonest and starts checking on KEYS he sold and if unused he sells them again. I doubt this will happen.
But a fresh Win 10 KEY from ebay is good after this upgrade period ends this January. Which might be BS anyhow. You make a Win 10 flash drive today via Microsoft download site that Paul Thurrott mentions (so do the ebay Win 10 KEY sellers) ........ I will say it just might work with a Win 7 or 8 KEY next year and any year.
But obviously with a few win 10 KEYS from ebay they should be good 5 years from now
I'm re-pinging because FReeper dennisw added some very useful comments here starting at comment #27 and running to the end of the thread, regarding how to install Win10 for very little cost, which I'm sure some of you will find interesting. Thank you dennisw for your work on this!