Got Windows 8 installed. The actual install (upgrading from Win 7 Ultimate), which most people breeze through, was pretty long and idiotic. Seems Win 8 Setup doesn't like the latest x64 CPUs / Mobos and AHCI SSD tech - I have one of the latest hexa-core intels, one i7 3930k with an SSD as primary... so yeah, major problems.
How this got past MS testing is beyond me. Anyway, after 3 days back and forth I managed to fix it by upgrading Mobo firmware and disabling AHCI (which disables some advanced SSD processes, but it's not noticeable).
Now that it's done, I really like the new Windows even though none of my monitors are touch enabled, contrary to some folks. Didn't lose anything with the upgrade process - which is amazing, it's like I just booted Win7, everything is the same with no need to reinstall anything except the usual driver updates.
In terms of performance, it's like a slightly more optimized Win7, although it's not very noticeable for me because I have a powerful system, thus Win7 was already very fast.
I don't find much to complain about. I installed Stardocks' Start8 and Decor8, getting the Start menu back (which I don't use much, but it's just $5) and am able to customize everything about the look, even in "Modern UI" mode. On the flipside, I can't really say "you need to upgrade" either. If you have Win7 without a touch monitor / laptop and don't want the Windows Store, Xbox Music, Smartglass, and a few minor things, there's really no need to upgrade.
Meanwhile, although Win8 doesn't ship with Tile customization options, there's already a small application for that, called OblyTile. I've been doing Tiles to use for some applications and games, feel free to make any request in the comments, and I'll do them.
Here's my current Windows 8 Start screen: