You've opened a can of worms here, lol.
I originally detested the MUI and used desktop exclusively.
This wasn't for lack of trying. I liked the idea of the MUI but I just felt my hands were tied too often.. for example, whenever I wanted to configure some behavior I sat there staring at the screen wondering where the hell to find the option - or if the option even existed. Often it didn't. If the option did exist, maybe it's accessed from swiping the top of the app, or maybe the bottom, or maybe Win+Z or Win+I.. or??? It was hard to tell.
At least with the old style apps you knew that if the app could do something, it was under a menu option. You might have to look through every menu but at least you know you were in the right spot and the option wasn't hiding behind some swipe gesture that was not obvious or consistent. Microsoft must address the "hidden options" problem. Either put a button on every app that somehow leads to all its options, or consistently force one swiping gesture to access all the apps options.
Now, having said that, now that I have my Sp3 I find myself using the MUI much more. When I'm using the SP3 as a tablet, I use MUI apps almost exclusively. I love snapping multiple apps to the screen, and as Frank said, the workflow for some apps is actually better in the MUI.
Here's one case in point that has me dancing in the street: I have the email app opened and click on a link in an email. IE is automatically snapped to half the screen to show me the link and my email is not closed and I don't have to figure out how to get back to my email app. I can just swipe to close IE and my email app returns to taking the whole screen. Nice. Compare this to how it worked on my ipad - click a link in an email and your email app disappears and the browser takes over the screen. When you're done looking at the browser content and want to return to your email, it felt like an unnecessary chore to get back to your email. MS nailed this one.
Also I like the search ability on the start screen and the ability to launch all apps there. It's sort of similar to the new interface that's showing up in a lot of the linux distro's... you can type a few keystrokes to find and launch an app, which is actually often much faster than using icons. Where MS has bungled it is there search is not smart. It only matches exact names, and it doesn't let me assign my own search terms. So if I want to start Oracle's Virtual Box, I can't just type in "vbox", I have to remember some part of the apps' actual name.