As my family and friends' "computer person", I decided to put Windows 10 on my PC (I back up all of my files on my NAS, so I didn't lose anything but moments of my life) so I could test it out before any of them got new prebuilts with Windows 10, or want to upgrade to the "best new thing". As it is right now, I WOULD NOT recommend you follow my example and upgrade to the new OS on your main computer. But first, the good. Here's what I like about Windows 10 Technical Preview:
1. It brought the start menu back. As such, I don't have to install Classic Shell on peoples' computers any more. Allowing the user to remove the tiles in the Start menu (Slimming it down considerably, and making it more like the classic Windows 98/2000 Start menu) seems like a nice touch, as well.
2. The integrated Search functionality
Being able to search the Internet straight from the Desktop seems to be rather useful. I'm honestly surprised it wasn't introduced sooner.
3. The Expose rip-off (don't remember the actual name), and virtual desktops.
Finally a replacement for Windows Vista/7's "Win+Tab" shortcut has came out. It's easy to use, and a bit better than what came with Windows 7 as it shows the entire window. I don't use virtual desktops (They seem pointless when I have two monitors, and typically when I switch between two workflows, I quit the applications related to one of the two), but I know that a lot of people like them (They were in a Windows XP PowerToy, IIRC).
4. Speed improvements
Windows 7 was a bit faster than Vista (Honestly, Moore's law is the only reason that 7 gained more acceptance than Vista), Windows 8 was a bit faster than Windows 7, and Windows 10 is a bit faster than Windows 8.
Now, we move on to things that I dislike/ would recommend people not install this on your physical computer because.
1: Driver incompatibilities.
I couldn't install Windows 8.1 drivers for my graphics card on Windows 10. I had to wait for Windows Update to pick it up, and even then I'm not sure that it actually installed the right driver.
2. Program incompatibilities.
The .Net framework won't install, I couldn't get any VMs to work, and I had at least one program complain that I wasn't running a new enough version of Windows for some reason.
3. General bugginess.
The first time I opened the Expose ripoff, my computer froze, I heard the Windows "device disconnected" noise, and my screen went black, before coming back to life 15 seconds later. When I uninstalled VirtualBox after it didn't work, it was still listed in the "Programs and Features" section in the Control Panel. When I tried to run it again, the computer blue-screened and restarted. Also, when I tried to uninstall Windows 8-style apps, I had to log out and log back in before I could see that they were gone from the start menu.
4. Lack of configurability
A big selling point with Windows 10 is that it brought the Start Menu back, and customizing it and resizing it is easy. However, I could only get links to the Control Panel and my Documents to display as links, not menus, I couldn't hide the dedicated Search and "Expose ripoff" buttons, and I couldn't resize the Start menu while it had apps in it horizontally- I only wanted a one-app thick bank of apps on the side of the Start menu, and the minimum thickness is apparently two.
5. Local accounts
In Windows 8, you could choose between setting up a local account, and a Microsoft account. In this one, the only way I could find to make a local account was to unplug my Ethernet cable during the OOBE, so it couldn't find Microsoft's servers and had to give me a local account.
In conclusion, I'm going back to Windows 7.