I upgraded from Kubuntu from 14.04 to 15.04 (in order to get MTP support for my phone), and unfortunately it is really beta quality.

• My settings from the previous version of KDE were not migrated.
• When opening a session, some programs are not restored (ex: Konsole). Super annoying.
• And also some windows are restore under my bottom bar.
• I get prompted to install Flash at each session.
• The auto-execution of programs at the session opening now only works for « programs » (having an associated « .desktop » file), not for shell scripts (though the configuration interface still allows adding such scripts). The list of the auto-executed programs is now located in « ~/.config/autostart » (and not anymore in « ~/.kde/Autstart »).
• Why is there two distinct directories for the thumbnails (~/.thumbnails and ~/
  .cache/thumbnails) ?
• The only way to add a program to a panel is by browsing the program hierarchy via the program menu widget (on the same panel). Searching a program by name will not allow you to add a program. Neither will the application launcher widget.
• In order to associate a keyboard short-cut to Klipper, I need two icons of Klipper in my bar.

I also noticed that the « open file » dialog boxes (in particular the one for choosing a wallpaper image) does not display preview images larger than 5 Mo (default limit). If you want to choose a photo taken with your DSLR it can be annoying. There is no global way to change this settings. To change it for a given application, open or create the configuration file « ~/.config/APPNAMErc » (for Kate: ~/.config/katerc ; for everything related to the desktop: ~/.config/plasmashellrc), and make sure the following section is present.

[PreviewSettings]
MaximumRemoteSize=0
MaximumSize=50000000
UseFileThumbnails=true

« MaximumSize » is the maximal size of the files. In this example: 50Mo.

My recommendation: if you want stability and the current version is satisfying for you, do not migrate.