For someone already running Windows 8 there's no reason not to upgrade -- Windows 8.1 is free, objectively and subjectively "better" than Windows 8, and doesn't represent any major paradigm shift.
As a Windows 7 user, though, it basically comes down to whether you need any of the 8/8.1 features, want to participate in the Windows Store ecosystem, and can live with the UI paradigm shift. That said, aside from being different, and sometimes a bit "in the way" by default, the new UI mode isn't bad in and of itself. Content-style applications (Games, media consumption, etc) in particular are perfectly at home in the Windows 8-style interface. For productivity apps like Word, excell, or VS, the desktop is still a better choice most times, although I can imagine, and there are starting to be, productivity apps in the Windows 8 style that are fluid and effective as long as they don't try to be all things to all people. When the hardware supports it, touch on a laptop or tablet is really nice -- its admittedly a bit forced on the desktop though. Some of the window 8 style touch gestures suck for mousing (I'm looking at you, drag-from-edge-to-X), but learning a couple keyboard shortcuts sidesteps the issue entirely.
If you're a Windows 7 user who skipped 8 because it didn't let you configure your machine to avoid the new stuff as much as you wanted, 8.1 goes even further to let you avoid it -- for example, you can boot straight into the desktop, and I think you can pin/launch windows 8-style apps from the desktop too -- then the Windows 8 start screen starts to feel less like a new environment, and more like a fancy start menu.
I was a bit resistant myself at first, but after using Windows 8 at home and at work for a month or so, and making a few tweaks, it feels as good and as fluid as Windows 7 did. I can tell because I recently upgraded to 8.1 and some settings seem to have reverted, and feel awkward again until I fix the setting.
On a technical level, both Windows 8 and Windows 8.1 are better than 7 in a lot of ways -- Your applications aren't suddenly going to run faster, but there's more and better hardware support, they consume fewer CPU and RAM resources (and also use excess RAM more effectively), boot more quickly (The new desktop I built boots from sleep in < 3 seconds, from power-off in about twice that), and the storage-spaces are pretty cool (you can pool drives together into one big space, and mark certain locations to be redundant in the case of a drive failure -- like a more-flexible RAID).