I did upgrade to Windows 8. However, I had prepared myself by reading a lot about it, and I had also run the Consumer Preview on my old PC, so I had a pretty good idea on what to expect.
The first thing you notice is obviously the new Start Screen. The first time you try to use it, it's quite weird, but I soon got accustomed to it.
The majority of preinstalled Modern UI apps aren't particularly useful, but Internet Explorer 10 might be the first version of IE worth using.
The People app is also nifty, it can use more accounts at once (for example, I set it to use my Facebook and Twitter accounts) and display all their notifications in one place. You may or may not like that - I found it useful.
Apps can also display data on their Start Screen tiles (which are called live tiles when they are set to update themselves). For example, the Weather tile shows the current weather and the Calendar tile shows the current date (well, I guess these two are obvious). Sadly, the People tile doesn't show updates, instead it displays a totally useless set of others' account pictures.
The Windows Store is quite new and there aren't nearly as many apps as on the iTunes store or iOS' App Store. The quality of the apps is lower as well. However, the majority of them is free, and there are a few good apps, too. I'm sure that, once more developers start making Windows 8 apps, this will change and there will be many more quality apps.
Another of the more useful apps is the Desktop, which isn't center stage anymore but you can still access it as a tile on the Start Screen. When you open it for the first time, you're greeted by your previous desktop, just as you'd expect. One of the things you'll notice is that Aero is gone, being replaced by a flat, Modern UI-esque theme. (oddly enough, the taskbar is still transparent, albeit with no blur). Again, you may like or dislike this change.
The desktop has been improved as well. There's a new task manager, which is much easier to use in the simple view, and is more powerful in the advanced view. File Explorer (the new Windows Explorer) uses the Ribbon interface, and a lot of the dialogs have been upgraded (for example, the Copy/Cut/Paste dialogs show a graph of the operation's speed and they allow you to pause it and resume it later).
The whole thing integrates nicely with your Microsoft account. You can sign in with it, and it can sync your Modern UI settings along your different computers. If you prefer, you can use a local account, but nothing will be synced and you won't be able to download Windows Store apps. You can, however, change to a Microsoft account at any time.
Most of your desktop applications will work on Windows 8 as well. Orbiter runs flawlessly. The OS boots up really quickly as well (to give you an idea, I used to put my PC to sleep so it would start faster. Now I shut it down, since booting is quicker than resuming from sleep). It's true that it's not exactly a 'complete' shutdown since they hibernate the Windows core now, but does it really matter?
tl;dr: In my opinion, Windows 8 is worth the $15 dollars they charge you for upgrading. While it does have some issues, there's nothing too serious, and I'd certainly recommend it.
One more thing. I have an Nvidia graphics card and I had some problems with the Modern UI at first: blurry text, black bands... If you face the same problem, just reset your graphics card's settings and it will be fixed.
