I love Windows 7, and I was hyped for Windows 10. So naturally, I was one of the first few people to reserve the upgrade when the icon appeared in my system tray - I was actually awake at the time. Two months rolled by, and I decided to, for the first time ever, use the built-in disk cleanup tool, which claimed it could free up over 42GB. Unfortunately, my filesystem was horribly corrupted and it destroyed my system beyond repair. I had to wipe my machine and reset it to its out-of-the-box state using the built-in system restore from the boot screen. Windows 10 hadn't launched yet, so I thought all I had to do was install a couple hundred updates and reserve my upgrade again.
Unfortunately, it turned out to be more complicated than that. Somehow by uninstalling the bloatware again, I bricked Windows Update - it wouldn't even open. Time for another few hours of resetting the system. Then, I installed 200+ updates and only had a few left, but somehow the BITS, the Background Intelligent Transfer Service that Windows Update uses to download updates, was broken. I spent 10+ hours trying everything I could find on the internet to fix it, to no avail. I had reserved my Windows 10 upgrade again, but since BITS was required to download it, I was screwed again.
By now Windows 10 had launched. I wiped my system again and installed the 200+ updates again. No Windows 10 icon in taskbar this time. I spent more hours trying various scripts and tricks from online to no avail. Finally, I found out about the manual upgrade process. I chose to not keep any data or software (so as to easily discard the bloatware) and finally, after four days, my computer was upgrading to Windows 10. I'm using it right now, and it's great.
That's not the end of the story though. Being that I'm still in college and it is between semesters, I live in a house with two other people who wanted to upgrade to Windows 10 - my mom and my sister. I'm the resident tech guy around here, so I was working on their computers in parallel to mine.
My sister's computer was relatively new, just bought a few months ago. When I ran sfc /scannow, it found corrupted files that it couldn't fix. I prayed that the upgrade wouldn't care, but I wasn't sure. As soon as the upgrade was made available, the icon vanished from her system tray. Again, I spent hours trying to make it come back, to no avail. Windows Update still claimed her update was reserved, but it wouldn't download it, and I feared the corrupted files were the issue. We backed up all her data, used the built-in factory reset, and had the same issue as me where after all the updates the icon would not appear. The manual upgrade did the trick.
My mom probably had the best experience out of all of us. The upgrade failed the first two times, and I feared it was because she also has corrupted system files that couldn't be fixed. However, some Googling online of the error code revealed that people had problems when upgrading to Windows 8/8.1 when there were corrupted user profiles or profiles on external drives. Since the computer used to be a shared computer, it had profiles for myself and my sister alongside my mom's. I deleted them, disconnected the external drive, and tried the upgrade again. Tada! It worked, and my mom was able to do an in-place upgrade, keeping all her files and software intact.
Moral of the story: if you have issues with the Windows 10 in-place upgrade, I strongly recommend just clouding all your data and doing the manual upgrade. If you don't mind reinstalling and redownloading everything, I suggest also choosing not to keep any data or software so that you can go with a pure vanilla Windows 10 installation (and leave behind whatever bloatware has been sitting on your computer for ages).
I don't offer technical support online or in the comments, by the way - you still need Google for that.