For me?
-The upgrade wizard was awful. Didn't pick up that almost all of my drivers were incompatible.
-Installing from flash drive proved much better, it was needed anyway
-Seemed jittery at first, but the updates are quickly improving it.
-No other compatibility issues thus far, and I run some pretty old shit. For those who have difficulty, Properties > Compatibility.
-My customers like it. Only met one person who wanted to revert, but I showed him some stuff, and he walked away happy with it.
-Cortana's goddamn useless. She seems to have a fetish for Bing. "Open Steam" was the ONLY command I issued her that didn't have some integration with bing.
-I don't like the revert back to the win7 style start menu. The start screen they have as a bunch of extra shit that interferes with keyboard navigation.
-The multiple desktops are actually pretty sweet. Never thought I'd use them because I'm on a multi-monitor setup.
-Some programs don't go back to desktop once they're run. I think this is because they're "splash screens."
-Bootup times are much faster than 8.
-Wasn't big on the new "photos" program they had to replace Slideshow. But you can revert.
-Seems no different than 8 when it comes to programming, image editing, video editing... So, yeah...
No real problems, but a lot of new features that they advertised, to me, is next to just about worthless. If you're iffy on getting it still, use the installer that comes with your legitimate copy of Windows. If you don't like it? you can revert. If you do? Format, and install 10 proper. The OS seems faster to boot, and although it has minor hitches for me, this is not a universal issue, and most people I talk to with prefabs or laptops say it's better all around. Compatibility with older software seems to be the issue, but this all boils down to lack of user knowledge. Some of the new features to bring the OS to be more "modern" are nice and helpful, but most of them are a blight, unwanted, and needlessly suck up resources doing nothing in their disabled state. If you have an XBOX, or a Windows phone, Windows 10 will want to integrate it's account across all platforms. If you're like me, with no consoles and an android phone, you will feel like a portion of the OS is simply not for you at all.
I still recommend getting it. If you have any idea what you're doing on a computer, you should still have the option to get it for free. C'mon. It's free, and it's an improvement.