Yes, I upgraded my Win 8.1 already. I actually didn’t expect I would qualify for the upgrade on July 29. It seems like being in Insider helps, even though I used Preview version in VM only.

I have summed my experience into couple of points below. It may be a long read for some, so TL;DR – my experience from upgrade is positive, Win 10 runs great and yes, I would recommend to upgrade.

1.  Upgrade itself was straightforward and simple. In my case it was just start it and let it be. The only input from my side was to go trought some settings at the end of installation (unless you decide to go with recommended settings, which is something, you most probably don’t want to do). I had in fact started the upgrade and went for a run and found new system installed on my return.
2. There was no immediate need to install any drivers after upgrade. I installed new Nvidia drivers just to be on safe side (Intel ones were updated automatically with the application that comes with them).
3. Logging in is the same as in Win 8.1 in my case. PIN settings remained the same.
4. Start. I didn’t have any issues with the one in Win 8.1, I may have liked it a bit. It is bit strange to get used to new one now. I changed it to full screen within minutes. I have not yet got used to the fact, that “shut down button” moved from top right corner to bottom left corner. And that its size is smaller.
5. All aplication I’ve yet used are working without any hiccups. Browsers, Office suite, Photoshop etc. I haven’t played any games nor run Steam or Origin yet. Even NEF codec is working.
6. I have mapped OneDrive, directly on the Internet, not the folder on system drive, as a network drive. Total Commander connects to it much faster compared to Win 8.1. It also feels more stable.
7. Look and feel has undergone a lot of changes. The title bar of application windows is white (unless the application allows you to change it to dark scheme). I haven’t found any setting within system to override this.
8. Language settings remained the same as in Win 8.1 contrary to what I’ve read somewhere. An English version of Win 8.1 originally came with my laptop, but I’ve since downloaded Czech language pack. Windows 10 installed in Czech (well, the settings you have to perform after installation – Ad ID etc. – was in English, but who cares).
9. Virtual desktops look great. Finally there is no need to use third-party software.
10. Window manipulation – “spliting” screen to show more application windows –  works better. Maybe because nothing slides from the sides.
11. We have got a couple of new shortcuts to control the new features like the already mentioned Virtual Desktops. Here is a list of 32 (not exclusively) new shortcuts. Have fun learning them all :-).
12. I find searching in Start faster and more accurate (I don’t event want to remember the accuracy, or better said inaccuracy in Win 8, a lot has been done in this area). In Taskbar there is even a button to begin a search applications, documents and setting, but honestly you don’t need to use it at all, just open Start and start typing.
13. Microsoft implemented new system sounds. Again something I will have to get used to to recognize what particular sounds mean.
14. The icons in Windows Explorer have been changed as we could see in Preview. They look flat. The fact is I do not care as I do not use Windows Explorer extensively, there has been no change in Total Commander.
15. File association can change for some kind of files, e.g. movie files are reassociated to built-in application (which does not support subtitels), the same happens for pictures etc., depending on how carefully you skip through the after installation settings.
16. The old Windows version and Windows 10 installation files are left on system drive (C:\Windows.old and C:\$Windows.~BT\ respectively). If you are sure you will not want to revert back to your previous Windows version, you can delete both folders and save yourself some space. In my case it was (I count both theses folders) 31,958 MB. A lot of space for SSD.

I will try to update this post later when and if I come across something.

I can say “Good work, MS.”

One last thing – when I was looking for whether I can upgrade or not I’ve found failed attempt to run the upgrade already on July 28, 23:33 GMT+2.

Update August 1, 2015
17. Applications “Photos” acts weirdly. When you open picture by double-clicking on it, you cannot get from that image to the previous or next one (that is when this application is the default on). When you launch the application first and then search for the picture, you want to see, it is possible to get from one picture to the other.