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I have been using Oxygen Transparent for about 24-30 months. The only problem i faced was some instability with the new 4.10 lock screen (so i had to add kscreenlocker or so to OpacityBlackList in Oxygen config). However, as I have mentioned, that was the only problem with this great project, so I consider it quite stable. It's much more lovely with blur than the default Oxygen theme, thus I'd like everyone to be able to choose this one without having to search for overlays, repositories, etc. Of course, there can still be some unresolved problems (I don't often read its changelogs), so I don't suggest making it default - just available out of the box.

Just to remind how it looks like (actually, there are no things breaking Oxygen interface ideas, like a border between window decoration and its content):
http://kde-look.org/CONTENT/content-pre3/127752-3.png