You know, I've been using Windows 8 since day one of beta development, from the early alpha build leaks, to the Developer Preview, Consumer, Release and RTM preview builds. The first word I said when I finished installing the Windows 8 Developer Preview was, "Whoa!" Since then, it's been an interesting ride that I enjoyed!
Honestly, there are a LOT of under the hood tweaks that makes Windows 8 flow like hot oil, especially if you have a SSD. It's fast and fluid, like Microsoft describes. With a mouse, some argue it's clunky and unintuitive to use. I personally use the Microsoft Touch Mouse, and I haven't physically clicked and dragged a window to snap it to the side in a LONG time. I did it the other day, it felt so clunky and odd since I've been using touch gestures to do that. Windows 8 in reality is pretty nice to use with a mouse. When you get a good hang of it, it just flows. That, along with a decently arranged Start Screen, multitasking becomes more efficient than it ever was with 7 and the start menu.
One of the things I do love about Windows 8 is how uber efficient it is over 7, both system resource wise and straight up electrically. It uses less energy to run the system, which in turn reduces power rates, and reduces heat output and slightly less stress on system components in the long run. From my time running 8, I haven't had ONE SINGLE SLOW DOWN. Not even one! My cold boot times have progressively gotten quicker, and stability is supreme. Before with 7, I'd had to do some advanced things to restore boot times as they progressively got slower and slower over time. I think that partly comes from the Automatic Maintenance feature of 8, as it runs the built in anti-virus scanner (Windows Defender), defrags the hard drive (I have it set to run EVERY day and it actually does where 7 did it when the hard drive was 10% fragmented), and does some other system tasks to keep it running smoothly. Action Center does a better job of alerting you of system warnings. For example, it'll tell you that startup entries are making it run slower than crap, then lets you open the new Task Manager to disable the ones you don't need starting up. With a third party anti-virus, it uses Action Center as a consolidated warning center to alert you of malware detection or outdated databases.
I personally love using the new metro apps in Windows 8, as I don't have to look like a dungeon troll with my Windows Phone in front of a MUCH more competent hardware that I can use to browse the same apps faster and better. I don't have to use two devices to browse different content, I can just use my PC to do both.
The design of the Start Screen to some is off-putting, but many seem to be enjoying it. It's a pretty flexible UI that can be what you make of it. Just need Desktop apps and items on it? Sure go ahead. Want some metro apps along side the Desktop apps? Yep! Just want metro apps? YES!
If you want to make the upgrade, there really isn't much against it really.
