I'm testing Kubuntu 15.04 for several days now (wanted to do it much earlier, but hit some compatibility issues). It's really impressive - the system is (again!) faster. Turns off so fast I am close to being worried something went wrong. Time it takes the OS to turn on is also very noticeably shorter.
But the biggest surprise for me is that... I like the new UI that comes with Plasma 5. In general I am not happy with the hype around "flat" design - I think it is less readable than an interface with shadows, gradients and clearly visible buttons. So I thought the new Kubuntu will not be a pleasant upgrade for me in this regard.
However, using the new Plasma turns out to be a real pleasure. It works great, looks good. And I've noticed several places have seen some major UI changes - mostly also for the good. The thing is more intuitive than it used to be, better organised and in general easy to use.
So, good job +Kubuntu
, good job +The KDE Community
PS. What does not work: I need to investigate it a bit more, but looks like 3D acceleration on VirtualBox is causing problems (parts of the GUI vanish), and so does installing nVidia drivers on a non-virtual machine (here I get a big load of crashes and a black screen).
PS2. I also miss window tabbing option, which is gone since 14.10 :-(