I have two laptops, one with windows 7 and another with 8.1. I updated the one with 8.1 to 10.
It updated to a black screen with cursor. After logging out and back in, the display showed fine, but I received a warning that the intel graphics drivers were no longer compatible with 10, which could lead to problems.
After installing the intel update utility, I managed to add compatible generic drivers for the laptop's graphics.
I have not installed much on the system so far aside from Libre Office and a few games for relations (Sims 3 for relation, and a few from GOG, which seem to be highly compatible).
The worst problem I have encountered to date was just over a week ago. The menu button suddenly stopped working. I could click on it, but nothing would happen. This also happened in 8.0. I was able to get it going again by logging out and back. Thereafter, a day later or so, whilst casually browsing explorer folders-the screen suddenly turned blank, and a message kept showing up from then on, telling me that cortana and the menu were not able to work. I could only click to sign out. Signing back in.. the same error again!
I was able to get the system working again by forcing an uninstall of Avast, which I had assumed to be the culprit, since it is rather heavy security software that locks down the system a fair bit. The Avast antivirus worked since installation. It is obvious that much security software is not fully compatible with 10 yet. I have switched to Panda antivirus with Glasswire gui for windows 10's own firewall since then, which are both pretty light, and have had no problems so far. Bitdefender free might also be a good option but I believe it's HIPS engine and (from what I have heard) inability to restore or allow false positives, might prove a problem on 10.
I have decided not to install any programs since, because 10 is still in it's first year. By agreeing to install windows 10 on it's first day of release, you are essentially a beta user. Windows is relying on it's users to report upcoming bugs in order to create a more stable system, and on developers to ensure their problems are compatible with 10's 'metro' (the tiled windows 8 style interface that 10 also uses for its menu and apps screen).
If you are going to use 10, try and keep your installations to a minimum, and before you update, make sure you back up ALL of your most important files and folders to another external hard drive, computer or at least dvd. One problem during installation is all it takes to mess up your system. PS installation may take a very long time - even if you think your system has frozen, be patient. If you want to continue using your pc as you are now and don't want to have to bother with potential issues, you might be best to stay with 7 or 8.1 for a few more months.
I have a link below on what many people are encountering with the menu freeze, a screenshot is half way down this page