Well, well, well. Just installed Windows 10 x64 upgrade over 8.1 on my HP ProBook 4540s Core i5 3rd gen lappy, and MAN OH MAN, it's smooth :D I've got a crappy 3Mbps connection, so it took a while to download, then had to wait for all the config stuff, but not one hiccup. Not one cause for concern. Not one error or stupid message. It just worked.
Very much into the new Start menu, the Task View (virtual desktops for you *nix nerds), Cortana speech recog, and enhanced window snap. The desktop is also quite a bit faster, even over 8.1. In general, the look is a lot cleaner in Windows apps. In desktop mode, even Modern app windows work like regular windows, so that the taskbar is always visible.
Since I use Chrome as my default browser, and many of my "apps" are just pinned websites, I found a weird behavior. Normally, when you pin a webpage to the Taskbar, its icon appears directly on the Taskbar. Not so, in Win10. The process happens, but the icon appears in the All Apps list in the new Start menu. From there you can pin it to the Start menu or the Taskbar.