I can say that I like the idea of preventing programs from running amok, but the execution is terrible. Apart from breaking legacy programs that want to set their own file associations (which is selective, it seems like programs can still set file associations during install but maybe not otherwise?), the thing that bugs me is that there are two different menus for setting file associations only one of which actually works. There's a "modern" metro style "default program" interface that often won't let you switch away from microsoft's apps, and then there's the old control panel method (which thankfully still works). It's another symptom of windows' growing pains.
I would normally just fix and forget once I found the way to fix it, but it bugs me because windows periodically resets the defaults to use its apps (so far it appears to happen every few months at major upgrades). It also seems to change lots of other settings at those upgrades (including privacy related settings, which is maddening). Changing my settings without asking me gets old really fast and is the single biggest reason I'm trying to migrate away from windows entirely.
If I'm willing to live with poorly documented, unpredictable, hidden UI, why would I pay Microsoft for it when I can get it for free without spyware on Linux