I've been testing Kubuntu 15.04 beta for a week and I'm very impressed with it. My only issues were:

-I've got an Assus laptop with an UEFI bios and there was no way to get installed the 15.04 from the USB disk. It started up ok but, at the moment of set up my disk partitions, it only created the Ext4 and the Swap. I had to install the 14.10 version (that created the Ext4, Swuap and EFI partitions) and, over it, made an online upgrade to 15.04.

-Nvidia 820M Optimus not working with 340_version propietary drivers. If I select them, I get the blank screen instead of login screen and I've got to switch to terminal to make the prime change to Intel graphics.

-Dropbox icon is not working on status bar. It's really annoying cause you don't receive notifications and, the worst thing, you can't control the dropbox menu.

-When I move the mouse pointer over an open menu of LibreOffice Writer, there is no selection bar moving through the menu options.

-I don't know if it's an issue but startup times are longer now that Kubuntu is installed on my laptop than when I started it from usb pendrive.

-At the beggining (two first days after instalation), the system didn't save my changes on Fonts and I had problems to switch off the system (the options in kde startup menu didn't work) and I had to do it using the terminal. After update the systems, those issues disappeared and now it's working ok.

-The Galician translation is not finished because I've got several texts in English.

By the moment, those are the only 'bad things'. I'm really happy with Kubuntu 15.04 and Plasma 5.2.2. By the way, Kubuntu is going to give us Plasma 5.3 in its final version?