A couple of days ago, Paul Thurrott wrote an editorial about Microsoft's Windows 10 reservation system. His point was that, while Microsoft claimed users of Windows 7 or 8.1 could upgrade on July 29th, they might not get it until later. Upgrades will start rolling out on the 29th of July, but the actual queue is expected to take several days. According to Microsoft's blog post, which shows blatant disrespect for the Oxford Comma, “Each day of the roll-out, we will listen, learn and update the experience for all Windows 10 users.”
Paul linked this backtrack to an episode of Seinfeld, one where Jerry reserves a rental car; his reservation was held, but a car was not. He stated that the availability date was clearly stated as July 29th, and not everyone will get it then. I can see his point, and I agree with it. Microsoft really should provide what they claim on the date that they claim it.
On the other hand, it is possible that Microsoft saw the whole reservation system as reserving your spot in line. That is, it might be that upgrade requests will be processed in reservation order, at least mostly, when devices are available. I imagine a “take a number” system where slots will be assigned for anyone below a threshold that increases as upgrades are fulfilled. Again, this is hypothetical, but I cannot really see any other reason for a reservation system in the first place, apart from pure marketing.
Either way, some may need to wait until after July 29th to experience Windows 10, and Microsoft botched their announcement.