That's a really, really difficult choice. Some people might judge by looks, but I've gone by familiarity here: for example, does it use established metaphors that a lot of users will be familiar with, or does it try and do new things?
I don't understand why a lot of operating systems can't evolve slowly, like OS X has done. Sure, the difference between OS 9 and OS X was massive, but we needed something new for so many reasons. And OS X has evolved slowly since then.
Why do a lot of operating systems feel like they need to throw away established workflows and metaphors (e.g. the desktop) and try something new? Just evolve what we already have. These metaphors are established because they work efficiently! Doing something new for the sake of doing something new is not a good idea. Believe me, I design user interfaces for a living! If you design a user interface - whether it's for an application or a web site - and you think "wow, I have an idea that nobody's ever done before"... Yeah, there's probably a good reason nobody's ever done that before. Why do you think cars still have pedals and steering wheels? Because although it takes effort to learn, it's the best way we have of controlling a big, fast metal box.
Usually, they do it to try and bring the same interface to the desktop, tablet, phone and TV. But these devices are very different in terms of processing power, screen size, how you hold/use them, and how you interact with them.
This is why I don't like Windows 8. I'm sitting in front of a desktop computer with a keyboard and mouse. Trying to do swipe gestures on a traditional two-buttons-and-a-scrollwheel mouse doesn't work. And who the hell decided that scrolling a mouse wheel up-and-down should make Metro scroll from side to side. And where can I get some of whatever they were smoking?
OS X does gestures much better with multi-finger swipe on the trackpad or magic mouse. It's so easy to flick between the desktop and full-screen apps.
Anywhere, that's enough of a rant. Here are my thoughts:
- iOS 7 isn't perfect. They'll get the look right eventually and it needs a better Music app. But it works in a similar way to iOS 6. So if you can use iOS 6, you'll be fine with iOS 7.
- Windows 8 is, to put it politely, a clusterf**k. It has a completely different interface to Windows 7. Sure, you can get back to the desktop, and computer-literate people can get used to it. But it still feels like two separate, disjointed interfaces slammed together. Also, forcing a touch-first interface on people using keyboards and mice wasn't a smart usability idea.
- I'm not a fan of Unity on the desktop, at all. We'll need to see how well it does on phones and tablets (and TVs!) later this year. But the interface is very inflexible. "You will have the dock on the left side of the screen because we say so." What if I like my dock across the bottom?
- Towards the end, I really liked Gnome 2. I thought it evolved into something really great. It didn't really need any effort to get used to. I could use it productively for day-to-day work. But Gnome 3 made a lot of mistakes. What little customisation Gnome 2 had was ripped out of Gnome 3. And... oh, no! I've accidentally moved my mouse into the top left corner again and everything's flown off the screen. I tried using it for my day-to-day work. I did not like it.
- I haven't used KDE since KDE 3.5. Tried KDE4, but I didn't like it. I've never really given it a chance since then, so it's unfair of me to comment on it.