No issues with the upgrade process. It felt like a regular WU session, just taking a little bit longer.
Upgraded a 2009 Dell XPS Studio laptop with a Celeron Duo, 3G RAM and an ATI Radeon HD 3670. Everything works and actually runs quite a bit faster than it did with Windows 7 on it.
Upgraded a 2010 XPS 7100 Dell desktop with an AMD Phenom 2, 12G RAM, an nVidia GTX 650 Ti, and it has an SSD. Everything works fine. I really haven't noticed a major improvement in speed, although it is a bit snappier, but I think that's due to using an SSD with a fairly powerful CPU.
Upgraded a 2013 Dell XPS 8700 desktop with an i5, 8G RAM, an nVidia GTX 960, and it also has an SSD. Everything works fine. Like the other XPS 7100, it is a bit snappier, but I haven't noticed a major increase in performance during normal operations. However, games appear to be sharper and perform noticeably better.
Upgraded a 2010 Dell Vostro, with a CORE, 2G RAM, and onboard graphics, running a 32 bit version of Windows 7 Pro. Everything works fine. The upgrade process with a bit slower. The computer runs quite a bit faster, even with over-multitasking the machine.
On my own system thus far, I am very pleased with Windows 10. The new options take a little bit getting used to, but not that much of a change from 8.1. I am having an issue with my system being part of a mysterious "organization" which controls some of my settings. I think I or someone else will resolve this issue, eventually, since I'm not the only one being controlled by The Organization. This minor detail isn't inhibiting the OS in any way, so I'm really not fussing over it. Aside from this, I haven't had any problems.
Although I haven't made an official switch from Firefox to MS Edge, I have set it up and imported my bookmarks. The browser is fast and streams video much better than Firefox using Silverlight. If it had Extension compatibility for my secure password management software, I'd use it all the time for watching movies via website streaming. Page loads are significantly faster, but I'd like the option of toggling Flash use for some web sites, rather than all or none.
I don't normally upgrade to the next OS when it first hits the street, but my time with Windows 10 as an Insider convinced me this one was worthy of immediate use.